diff --git a/changelog/duplicate-implementations-deprecation.dd b/changelog/duplicate-implementations-deprecation.dd
new file mode 100644
index 000000000000..2893c192cad4
--- /dev/null
+++ b/changelog/duplicate-implementations-deprecation.dd
@@ -0,0 +1,23 @@
+Usage of identical functions in a single module
+
+A binary should never contain multiple versions of a symbol.
+Before this version, it was allowed to provide multiple implementation for a function,
+even if they result in the same mangling, which can result in undefined behavior:
+
+---
+extern(C):
+void foo(int) { }
+void foo(double) { } // will error now
+---
+
+However, it's still allowed to have multiple declarations as long as there's at
+most one implementation:
+
+---
+extern(C):
+void foo(int) { }
+void foo(double);
+---
+
+Note that this change is only relevant for mangling schemes that do no support
+overloading like `extern(C)`.
diff --git a/src/dmd/semantic2.d b/src/dmd/semantic2.d
index 768422dd4b1b..57172c374148 100644
--- a/src/dmd/semantic2.d
+++ b/src/dmd/semantic2.d
@@ -357,9 +357,6 @@ 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)
         {
             OutBuffer buf1;
@@ -413,7 +410,8 @@ private extern(C++) final class Semantic2Visitor : Visitor
 
                     auto tf1 = cast(TypeFunction)f1.type;
                     auto tf2 = cast(TypeFunction)f2.type;
-                    error(f2.loc, "%s `%s%s` cannot be overloaded with %s`extern(%s)` function at %s",
+                    // @@@DEPRECATED_2.092@@@
+                    f2.deprecation("%s `%s%s` cannot be overloaded with %s`extern(%s)` function at %s",
                             f2.kind(),
                             f2.toPrettyChars(),
                             parametersTypeToChars(tf2.parameters, tf2.varargs),
@@ -434,7 +432,7 @@ private extern(C++) final class Semantic2Visitor : Visitor
                 if (strcmp(s1, s2) == 0)
                 {
                     auto tf2 = cast(TypeFunction)f2.type;
-                    error(f2.loc, "%s `%s%s` conflicts with previous declaration at %s",
+                    f2.error("%s `%s%s` conflicts with previous declaration at %s",
                             f2.kind(),
                             f2.toPrettyChars(),
                             parametersTypeToChars(tf2.parameters, tf2.varargs),
diff --git a/test/fail_compilation/fail17492.d b/test/fail_compilation/fail17492.d
index dca6f6fc0888..85af4916cefa 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` function `fail17492.C.testE()` conflicts with previous declaration at fail_compilation/fail17492.d(13)
+fail_compilation/fail17492.d(37): Error: function `fail17492.S.testE` 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..9dccb46d0c5b 100644
--- a/test/fail_compilation/fail2789.d
+++ b/test/fail_compilation/fail2789.d
@@ -1,11 +1,18 @@
 /*
-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
+REQUIRED_ARGS: -de
 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)
+fail_compilation/fail2789.d(25): Error: function `fail2789.A2789.m` function `fail2789.A2789.m()` conflicts with previous declaration at fail_compilation/fail2789.d(20)
+fail_compilation/fail2789.d(35): Error: function `fail2789.A2789.m` function `fail2789.A2789.m()` conflicts with previous declaration at fail_compilation/fail2789.d(20)
+fail_compilation/fail2789.d(47): Error: function `fail2789.f3` function `fail2789.f3()` conflicts with previous declaration at fail_compilation/fail2789.d(46)
+fail_compilation/fail2789.d(50): Error: function `fail2789.f4` function `fail2789.f4()` conflicts with previous declaration at fail_compilation/fail2789.d(49)
+fail_compilation/fail2789.d(53): Error: function `fail2789.f5` function `fail2789.f5()` conflicts with previous declaration at fail_compilation/fail2789.d(52)
+fail_compilation/fail2789.d(56): Error: function `fail2789.f6` function `fail2789.f6()` conflicts with previous declaration at fail_compilation/fail2789.d(55)
+fail_compilation/fail2789.d(60): Deprecation: function `fail2789.f_ExternC1` function `fail2789.f_ExternC1()` cannot be overloaded with another `extern(C)` function at fail_compilation/fail2789.d(59)
+fail_compilation/fail2789.d(63): Deprecation: function `fail2789.f_ExternC2` function `fail2789.f_ExternC2(int)` cannot be overloaded with another `extern(C)` function at fail_compilation/fail2789.d(62)
+fail_compilation/fail2789.d(66): Deprecation: function `fail2789.f_ExternC3` function `fail2789.f_ExternC3()` cannot be overloaded with another `extern(C)` function at fail_compilation/fail2789.d(65)
+fail_compilation/fail2789.d(69): Error: function `fail2789.f_MixExtern1` function `fail2789.f_MixExtern1()` conflicts with previous declaration at fail_compilation/fail2789.d(68)
+fail_compilation/fail2789.d(90): Error: function `fail2789.mul14147` function `fail2789.mul14147(const(int[]) left, const(int[]) right)` conflicts with previous declaration at fail_compilation/fail2789.d(86)
 ---
 */
 class A2789
@@ -30,15 +37,6 @@ 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
 
@@ -57,15 +55,7 @@ void f5() @system {}    // conflict
 auto f6() { return 10; }    // int()
 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(76): Error: function `fail2789.f_MixExtern1()` conflicts with previous declaration at fail_compilation/fail2789.d(75)
----
-*/
+
 extern(C) void f_ExternC1() {}
 extern(C) void f_ExternC1() {}      // conflict
 
@@ -90,12 +80,6 @@ extern (C) void f_ExternC5(int sig) @nogc;      // no error
 extern (C) void f_ExternC6(int sig);
 extern (C) void f_ExternC6(int sig) @nogc {}    // no error
 
-/*
-TEST_OUTPUT:
----
-fail_compilation/fail2789.d(103): Error: function `fail2789.mul14147(const(int[]) left, const(int[]) right)` conflicts with previous declaration at fail_compilation/fail2789.d(99)
----
-*/
 struct S14147(alias func)
 {
 }
diff --git a/test/fail_compilation/fail5634.d b/test/fail_compilation/fail5634.d
index 0044adf8df29..9c962af329b7 100644
--- a/test/fail_compilation/fail5634.d
+++ b/test/fail_compilation/fail5634.d
@@ -1,3 +1,10 @@
+// REQUIRED_ARGS: -de
+/*
+TEST_OUTPUT:
+---
+fail_compilation/fail5634.d(9): Error: function `D main` function `D main()` conflicts with previous declaration at fail_compilation/fail5634.d(8)
+---
+*/
 void main() { }
 void main() { }
 
diff --git a/test/fail_compilation/fail7443.d b/test/fail_compilation/fail7443.d
index 0fb165d5dcc8..4673e491d32d 100644
--- a/test/fail_compilation/fail7443.d
+++ b/test/fail_compilation/fail7443.d
@@ -2,8 +2,9 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/fail7443.d(12): Deprecation: `static` has no effect on a constructor inside a `static` block. Use `static this()`
-fail_compilation/fail7443.d(13): Deprecation: `shared static` has no effect on a constructor inside a `shared static` block. Use `shared static this()`
+fail_compilation/fail7443.d(13): Deprecation: `static` has no effect on a constructor inside a `static` block. Use `static this()`
+fail_compilation/fail7443.d(14): Deprecation: `shared static` has no effect on a constructor inside a `shared static` block. Use `shared static this()`
+fail_compilation/fail7443.d(15): Error: constructor `fail7443.Foo.this` constructor `fail7443.Foo.this()` conflicts with previous declaration at fail_compilation/fail7443.d(13)
 ---
 */
 
diff --git a/test/fail_compilation/test18282.d b/test/fail_compilation/test18282.d
index 8e7832ce14ec..cd60daf5f6c2 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..6db4d8a732c5
--- /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` function `test18385.foo(double)` cannot be overloaded with another `extern(C)` function at fail_compilation/test18385.d(12)
+fail_compilation/test18385.d(18): Deprecation: function `test18385.S.foo` function `test18385.S.foo(double)` cannot be overloaded with another `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() { }