diff --git a/changelog/duplicate-implementations-deprecation.dd b/changelog/duplicate-implementations-deprecation.dd
new file mode 100644
index 000000000000..c296839dc458
--- /dev/null
+++ b/changelog/duplicate-implementations-deprecation.dd
@@ -0,0 +1,33 @@
+Diagnostics for conflicting function definitions within a module
+
+Previously, multiple definitions of identical functions within a module were not
+recognized, although they have the same mangling. This was problematic because a
+binary cannot contain multiple definitions of a symbol, which caused undefined
+behavior depending on the compiler backend.
+
+DMD will now raise an error message if there are conflicting implementations
+within a single module:
+
+---
+void foo() {}
+void foo() {} // error
+---
+
+Multiple declarations are still allowed as long as there is at most one definition:
+
+---
+void bar(int);
+void bar(int) { }
+void bar(int);
+---
+
+DMD will issue a deprecation for mangling schemes that don't support overloading
+(`extern(C|Windows|System)`):
+
+---
+extern(C):
+void foo(int) { }
+void foo(double) { } // deprecation
+---
+
+This deprecation will become an error in 2.105.
diff --git a/src/dmd/frontend.h b/src/dmd/frontend.h
index 37f92bc9d32d..142af85b03cb 100644
--- a/src/dmd/frontend.h
+++ b/src/dmd/frontend.h
@@ -5388,6 +5388,7 @@ class TypeFunction : public TypeNext
     bool isInOutQual() const;
     void isInOutQual(bool v);
     bool iswild() const;
+    bool attributesEqual(const TypeFunction* const other) const;
     void accept(Visitor* v);
 };
 
diff --git a/src/dmd/mtype.d b/src/dmd/mtype.d
index 4f2ea08bdd6d..8171b6d7f522 100644
--- a/src/dmd/mtype.d
+++ b/src/dmd/mtype.d
@@ -5198,6 +5198,18 @@ extern (C++) final class TypeFunction : TypeNext
         return (funcFlags & (FunctionFlag.inoutParam | FunctionFlag.inoutQual)) != 0;
     }
 
+    /// Returns: whether `this` function type has the same attributes (`@safe`,...) as `other`
+    bool attributesEqual(const scope TypeFunction other) const pure nothrow @safe @nogc
+    {
+        enum attributes = FunctionFlag.isnothrow
+                        | FunctionFlag.isnogc
+                        | FunctionFlag.islive;
+
+        return this.trust == other.trust &&
+                this.purity == other.purity &&
+                (this.funcFlags & attributes) == (other.funcFlags & attributes);
+    }
+
     override void accept(Visitor v)
     {
         v.visit(this);
@@ -6674,6 +6686,33 @@ extern (C++) struct ParameterList
     {
         return ParameterList(Parameter.arraySyntaxCopy(parameters), varargs);
     }
+
+    /// Compares this to another ParameterList (and expands tuples if necessary)
+    extern (D) bool opEquals(scope ref ParameterList other) const
+    {
+        if (stc != other.stc || varargs != other.varargs || (!parameters != !other.parameters))
+            return false;
+
+        if (this.parameters is other.parameters)
+            return true;
+
+        size_t idx;
+        bool diff;
+
+        // Pairwise compare each parameter
+        // Can this avoid the O(n) indexing for the second list?
+        foreach (_, p1; cast() this)
+        {
+            auto p2 = other[idx++];
+            if (!p2 || p1 != p2) {
+                diff = true;
+                break;
+            }
+        }
+
+        // Ensure no remaining parameters in `other`
+        return !diff && other[idx] is null;
+    }
 }
 
 
diff --git a/src/dmd/semantic2.d b/src/dmd/semantic2.d
index 266fe16c2d0c..ffb2f1515563 100644
--- a/src/dmd/semantic2.d
+++ b/src/dmd/semantic2.d
@@ -359,17 +359,19 @@ private extern(C++) final class Semantic2Visitor : Visitor
 
         //printf("FuncDeclaration::semantic2 [%s] fd0 = %s %s\n", loc.toChars(), toChars(), type.toChars());
 
-        // https://issues.dlang.org/show_bug.cgi?id=18385
-        // Disable for 2.079, s.t. a deprecation cycle can be started with 2.080
-        if (0)
-        if (fd.overnext && !fd.errors)
+        // Only check valid functions which have a body to avoid errors
+        // for multiple declarations, e.g.
+        // void foo();
+        // void foo();
+        if (fd.fbody && fd.overnext && !fd.errors)
         {
             OutBuffer buf1;
             OutBuffer buf2;
 
             // Always starts the lookup from 'this', because the conflicts with
             // previous overloads are already reported.
-            auto f1 = fd;
+            alias f1 = fd;
+            auto tf1 = cast(TypeFunction) f1.type;
             mangleToFuncSignature(buf1, f1);
 
             overloadApply(f1, (Dsymbol s)
@@ -379,7 +381,7 @@ private extern(C++) final class Semantic2Visitor : Visitor
                     return 0;
 
                 // Don't have to check conflict between declaration and definition.
-                if ((f1.fbody !is null) != (f2.fbody !is null))
+                if (f2.fbody is null)
                     return 0;
 
                 /* Check for overload merging with base class member functions.
@@ -393,35 +395,26 @@ private extern(C++) final class Semantic2Visitor : Visitor
                 if (f1.overrides(f2))
                     return 0;
 
+                auto tf2 = cast(TypeFunction) f2.type;
+
                 // extern (C) functions always conflict each other.
                 if (f1.ident == f2.ident &&
                     f1.toParent2() == f2.toParent2() &&
                     (f1.linkage != LINK.d && f1.linkage != LINK.cpp) &&
-                    (f2.linkage != LINK.d && f2.linkage != LINK.cpp))
+                    (f2.linkage != LINK.d && f2.linkage != LINK.cpp) &&
+
+                    // But allow the hack to declare overloads with different parameters/STC's
+                    (!tf1.attributesEqual(tf2) || tf1.parameterList != tf2.parameterList))
                 {
-                    /* Allow the hack that is actually used in druntime,
-                     * to ignore function attributes for extern (C) functions.
-                     * TODO: Must be reconsidered in the future.
-                     *  BUG: https://issues.dlang.org/show_bug.cgi?id=18206
-                     *
-                     *  extern(C):
-                     *  alias sigfn_t  = void function(int);
-                     *  alias sigfn_t2 = void function(int) nothrow @nogc;
-                     *  sigfn_t  bsd_signal(int sig, sigfn_t  func);
-                     *  sigfn_t2 bsd_signal(int sig, sigfn_t2 func) nothrow @nogc;  // no error
-                     */
-                    if (f1.fbody is null || f2.fbody is null)
-                        return 0;
-
-                    auto tf2 = cast(TypeFunction)f2.type;
-                    error(f2.loc, "%s `%s%s` cannot be overloaded with %s`extern(%s)` function at %s",
-                            f2.kind(),
-                            f2.toPrettyChars(),
-                            parametersTypeToChars(tf2.parameterList),
-                            (f1.linkage == f2.linkage ? "another " : "").ptr,
-                            linkageToChars(f1.linkage), f1.loc.toChars());
-                    f2.type = Type.terror;
-                    f2.errors = true;
+                    // @@@DEPRECATED_2.094@@@
+                    // Deprecated in 2020-08, make this an error in 2.104
+                    f2.deprecation("cannot overload `extern(%s)` function at %s",
+                            linkageToChars(f1.linkage),
+                            f1.loc.toChars());
+
+                    // Enable this when turning the deprecation into an error
+                    // f2.type = Type.terror;
+                    // f2.errors = true;
                     return 0;
                 }
 
@@ -434,7 +427,6 @@ private extern(C++) final class Semantic2Visitor : Visitor
                 //printf("+%s\n\ts1 = %s\n\ts2 = %s @ [%s]\n", toChars(), s1, s2, f2.loc.toChars());
                 if (strcmp(s1, s2) == 0)
                 {
-                    auto tf2 = cast(TypeFunction)f2.type;
                     error(f2.loc, "%s `%s%s` conflicts with previous declaration at %s",
                             f2.kind(),
                             f2.toPrettyChars(),
diff --git a/test/compilable/test18385.d b/test/compilable/test18385.d
new file mode 100644
index 000000000000..5c789cb835cf
--- /dev/null
+++ b/test/compilable/test18385.d
@@ -0,0 +1,42 @@
+/*
+Reduced from the assertion failure in the glue layer when compiling DWT.
+A `compilable` test because it needs codegen.
+
+Remove this test once the deprecation for conflicting deprecations ends,
+see visit(FuncDeclaration) in semantic2.d for details.
+
+TEST_OUTPUT:
+---
+compilable/test18385.d(23): Deprecation: function `test18385.is_paragraph_start` cannot overload `extern(C)` function at compilable/test18385.d(22)
+compilable/test18385.d(26): Deprecation: function `test18385.foo` cannot overload `extern(C)` function at compilable/test18385.d(25)
+compilable/test18385.d(29): Deprecation: function `test18385.trust` cannot overload `extern(C)` function at compilable/test18385.d(28)
+compilable/test18385.d(32): Deprecation: function `test18385.purity` cannot overload `extern(C)` function at compilable/test18385.d(31)
+compilable/test18385.d(35): Deprecation: function `test18385.nogc` cannot overload `extern(C)` function at compilable/test18385.d(34)
+compilable/test18385.d(38): Deprecation: function `test18385.nothrow_` cannot overload `extern(C)` function at compilable/test18385.d(37)
+compilable/test18385.d(41): Deprecation: function `test18385.live` cannot overload `extern(C)` function at compilable/test18385.d(40)
+---
+*/
+
+extern(C)
+{
+	uint is_paragraph_start(){ return 0; }
+	uint is_paragraph_start(int){ return 0; }
+
+	void foo(char, bool) {}
+	void foo(byte, char) {}
+
+	void trust() {}
+	void trust() @safe {}
+
+	void purity() {}
+	void purity() pure {}
+
+	void nogc() {}
+	void nogc() @safe {}
+
+	void nothrow_() {}
+	void nothrow_() nothrow {}
+
+	void live() {}
+	void live() @live {}
+}
diff --git a/test/fail_compilation/fail17492.d b/test/fail_compilation/fail17492.d
index dca6f6fc0888..7236c22a8bcf 100644
--- a/test/fail_compilation/fail17492.d
+++ b/test/fail_compilation/fail17492.d
@@ -1,11 +1,9 @@
 /*
-https://issues.dlang.org/show_bug.cgi?id=18385
-Disabled for 2.079, s.t. a deprecation cycle can be started with 2.080
-DISABLED: win32 win64 osx linux freebsd dragonflybsd
+REQUIRED_ARGS: -de
 TEST_OUTPUT:
 ---
-fail_compilation/fail17492.d(20): Error: class `fail17492.C.testE.I` already exists at fail17492.d(13). Perhaps in another function with the same name?
-fail_compilation/fail17492.d(37): Error: struct `fail17492.S.testE.I` already exists at fail17492.d(30). Perhaps in another function with the same name?
+fail_compilation/fail17492.d(20): Error: function `fail17492.C.testE()` conflicts with previous declaration at fail_compilation/fail17492.d(13)
+fail_compilation/fail17492.d(37): Error: function `fail17492.S.testE()` conflicts with previous declaration at fail_compilation/fail17492.d(30)
 ---
 https://issues.dlang.org/show_bug.cgi?id=17492
 */
diff --git a/test/fail_compilation/fail2789.d b/test/fail_compilation/fail2789.d
index ef9d858fcaab..4e9f2c416152 100644
--- a/test/fail_compilation/fail2789.d
+++ b/test/fail_compilation/fail2789.d
@@ -1,13 +1,14 @@
 /*
-DISABLED: win32 win64 osx linux freebsd dragonflybsd
 https://issues.dlang.org/show_bug.cgi?id=18385
-Disabled for 2.079, s.t. a deprecation cycle can be started with 2.080
+
 TEST_OUTPUT:
 ---
 fail_compilation/fail2789.d(15): Error: function `fail2789.A2789.m()` conflicts with previous declaration at fail_compilation/fail2789.d(10)
 fail_compilation/fail2789.d(25): Error: function `fail2789.A2789.m()` conflicts with previous declaration at fail_compilation/fail2789.d(10)
 ---
 */
+#line 7
+
 class A2789
 {
     int m()
@@ -33,12 +34,12 @@ class A2789
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/fail2789.d(46): Error: function `fail2789.f3()` conflicts with previous declaration at fail_compilation/fail2789.d(45)
 fail_compilation/fail2789.d(49): Error: function `fail2789.f4()` conflicts with previous declaration at fail_compilation/fail2789.d(48)
 fail_compilation/fail2789.d(52): Error: function `fail2789.f5()` conflicts with previous declaration at fail_compilation/fail2789.d(51)
 fail_compilation/fail2789.d(55): Error: function `fail2789.f6()` conflicts with previous declaration at fail_compilation/fail2789.d(54)
 ---
 */
+
 void f1();
 void f1() {}    // ok
 
@@ -60,9 +61,9 @@ auto f6() { return ""; }    // string(), conflict
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/fail2789.d(67): Error: function `fail2789.f_ExternC1()` cannot be overloaded with another `extern(C)` function at fail_compilation/fail2789.d(66)
-fail_compilation/fail2789.d(70): Error: function `fail2789.f_ExternC2(int)` cannot be overloaded with another `extern(C)` function at fail_compilation/fail2789.d(69)
-fail_compilation/fail2789.d(73): Error: function `fail2789.f_ExternC3()` cannot be overloaded with another `extern(C)` function at fail_compilation/fail2789.d(72)
+fail_compilation/fail2789.d(67): Error: function `fail2789.f_ExternC1()` conflicts with previous declaration at fail_compilation/fail2789.d(66)
+fail_compilation/fail2789.d(70): Deprecation: function `fail2789.f_ExternC2` cannot overload `extern(C)` function at fail_compilation/fail2789.d(69)
+fail_compilation/fail2789.d(73): Deprecation: function `fail2789.f_ExternC3` cannot overload `extern(C)` function at fail_compilation/fail2789.d(72)
 fail_compilation/fail2789.d(76): Error: function `fail2789.f_MixExtern1()` conflicts with previous declaration at fail_compilation/fail2789.d(75)
 ---
 */
diff --git a/test/fail_compilation/fail47.d b/test/fail_compilation/fail47.d
index 4da1926dfec7..9da0c0927a28 100644
--- a/test/fail_compilation/fail47.d
+++ b/test/fail_compilation/fail47.d
@@ -1,12 +1,7 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/fail47.d(13): Error: variable `fail47._foo` is aliased to a function
-fail_compilation/fail47.d(13): Error: variable `fail47._foo` is aliased to a function
-fail_compilation/fail47.d(13): Error: variable `fail47._foo` is aliased to a function
-fail_compilation/fail47.d(18): Error: none of the overloads of `foo` are callable using argument types `(int)`, candidates are:
-fail_compilation/fail47.d(12):        `fail47.foo()`
-fail_compilation/fail47.d(13): Error: variable `fail47._foo` is aliased to a function
+fail_compilation/fail47.d(8): Error: variable `fail47._foo` is aliased to a function
 ---
 */
 void foo() {}
diff --git a/test/fail_compilation/fail5634.d b/test/fail_compilation/fail5634.d
index 87943f418756..e2442e65593f 100644
--- a/test/fail_compilation/fail5634.d
+++ b/test/fail_compilation/fail5634.d
@@ -1,8 +1,8 @@
 /*
 TEST_OUTPUT:
 ----
-fail_compilation/fail5634.d(9): Error: only one `main`$?:windows=, `WinMain`, or `DllMain`$ allowed. Previously found `main` at fail_compilation/fail5634.d(8)
-----
+fail_compilation/fail5634.d(9): Error: function `D main()` conflicts with previous declaration at fail_compilation/fail5634.d(8)
+---
 */
 
 void main() { }
diff --git a/test/fail_compilation/fail7443.d b/test/fail_compilation/fail7443.d
index 3770879e7917..2c5c94007462 100644
--- a/test/fail_compilation/fail7443.d
+++ b/test/fail_compilation/fail7443.d
@@ -10,5 +10,5 @@ class Foo
 {
     public static { this() {}}
     public shared static { this() {}}
-    public {this() {}}
+    public {this(int) {}}
 }
diff --git a/test/fail_compilation/test18282.d b/test/fail_compilation/test18282.d
index 72190410f9d8..b85380163e50 100644
--- a/test/fail_compilation/test18282.d
+++ b/test/fail_compilation/test18282.d
@@ -46,7 +46,7 @@ S2000 bar2()
 }
 
 
-void bar2()
+void bar3()
 {
     int i;
     char c;
diff --git a/test/fail_compilation/test18385.d b/test/fail_compilation/test18385.d
new file mode 100644
index 000000000000..3ec8253bd627
--- /dev/null
+++ b/test/fail_compilation/test18385.d
@@ -0,0 +1,31 @@
+/*
+REQUIRED_ARGS: -de
+TEST_OUTPUT:
+---
+fail_compilation/test18385.d(13): Deprecation: function `test18385.foo` cannot overload `extern(C)` function at fail_compilation/test18385.d(12)
+fail_compilation/test18385.d(18): Deprecation: function `test18385.S.foo` cannot overload `extern(C)` function at fail_compilation/test18385.d(17)
+---
+*/
+
+extern (C):
+
+void foo(int) { }
+void foo(double) { }
+
+struct S
+{
+    static void foo(int) {}
+    static void foo(double) {}
+}
+
+void foo2(int) { }
+extern(D) void foo2(double) { } // OK as it has a different mangling
+
+void foo3(int) { }
+void foo3(double); // duplicate declarations are allowed
+
+void foo4();
+void foo4() { }
+
+extern(D) void foo5();
+extern(D) void foo5() { }