diff --git a/mypyc/irbuild/builder.py b/mypyc/irbuild/builder.py
index 573ca334a5d1..5ed617aa925f 100644
--- a/mypyc/irbuild/builder.py
+++ b/mypyc/irbuild/builder.py
@@ -175,6 +175,7 @@ def __init__(
         self.graph = graph
         self.ret_types: list[RType] = []
         self.functions: list[FuncIR] = []
+        self.function_names: set[tuple[str | None, str]] = set()
         self.classes: list[ClassIR] = []
         self.final_names: list[tuple[str, RType]] = []
         self.callable_class_names: set[str] = set()
@@ -1326,6 +1327,14 @@ def error(self, msg: str, line: int) -> None:
     def note(self, msg: str, line: int) -> None:
         self.errors.note(msg, self.module_path, line)
 
+    def add_function(self, func_ir: FuncIR, line: int) -> None:
+        name = (func_ir.class_name, func_ir.name)
+        if name in self.function_names:
+            self.error(f'Duplicate definition of "{name[1]}" not supported by mypyc', line)
+            return
+        self.function_names.add(name)
+        self.functions.append(func_ir)
+
 
 def gen_arg_defaults(builder: IRBuilder) -> None:
     """Generate blocks for arguments that have default values.
diff --git a/mypyc/irbuild/function.py b/mypyc/irbuild/function.py
index ebf7fa9a54de..b1785f40550e 100644
--- a/mypyc/irbuild/function.py
+++ b/mypyc/irbuild/function.py
@@ -103,7 +103,7 @@ def transform_func_def(builder: IRBuilder, fdef: FuncDef) -> None:
     if func_reg:
         builder.assign(get_func_target(builder, fdef), func_reg, fdef.line)
     maybe_insert_into_registry_dict(builder, fdef)
-    builder.functions.append(func_ir)
+    builder.add_function(func_ir, fdef.line)
 
 
 def transform_overloaded_func_def(builder: IRBuilder, o: OverloadedFuncDef) -> None:
diff --git a/mypyc/test-data/irbuild-statements.test b/mypyc/test-data/irbuild-statements.test
index 490b41336e88..b7c67730a05f 100644
--- a/mypyc/test-data/irbuild-statements.test
+++ b/mypyc/test-data/irbuild-statements.test
@@ -1123,3 +1123,27 @@ L6:
     r14 = CPy_NoErrOccured()
 L7:
     return 1
+
+[case testConditionalFunctionDefinition]
+if int():
+    def foo() -> int:
+        return 0
+else:
+    def foo() -> int:  # E
+        return 1
+
+def bar() -> int:
+    return 0
+
+if int():
+    def bar() -> int:  # E
+        return 1
+[out]
+main:5: error: Duplicate definition of "foo" not supported by mypyc
+main:12: error: Duplicate definition of "bar" not supported by mypyc
+
+[case testRepeatedUnderscoreFunctions]
+def _(arg): pass
+def _(arg): pass
+[out]
+main:2: error: Duplicate definition of "_" not supported by mypyc
diff --git a/mypyc/test-data/run-misc.test b/mypyc/test-data/run-misc.test
index c40e0fc55f0e..f77ba3a1302b 100644
--- a/mypyc/test-data/run-misc.test
+++ b/mypyc/test-data/run-misc.test
@@ -1117,10 +1117,6 @@ for _ in range(2):
     except AssertionError:
         pass
 
-[case testRepeatedUnderscoreFunctions]
-def _(arg): pass
-def _(arg): pass
-
 [case testUnderscoreFunctionsInMethods]
 
 class A: