diff --git a/src/compiler.ts b/src/compiler.ts
index 36f09ab8aa..af751d336c 100644
--- a/src/compiler.ts
+++ b/src/compiler.ts
@@ -7186,6 +7186,21 @@ export class Compiler extends DiagnosticEmitter {
     let sourceFunction = flow.sourceFunction;
     let isNamed = declaration.name.text.length > 0;
     let isSemanticallyAnonymous = !isNamed || contextualType != Type.void;
+
+    // Check for duplicate named function declarations before creating the
+    // prototype, since registering a concrete element with a duplicate
+    // internalName would trigger an assertion error.
+    if (!isSemanticallyAnonymous) {
+      let existingLocal = flow.getScopedLocal(declaration.name.text);
+      if (existingLocal) {
+        this.error(
+          DiagnosticCode.Duplicate_identifier_0,
+          declaration.name.range, declaration.name.text
+        );
+        return this.module.unreachable();
+      }
+    }
+
     let prototype = new FunctionPrototype(
       isSemanticallyAnonymous
         ? `${isNamed ? declaration.name.text : "anonymous"}|${sourceFunction.nextAnonymousId++}`
diff --git a/tests/compiler/duplicate-function-in-scope.json b/tests/compiler/duplicate-function-in-scope.json
new file mode 100644
index 0000000000..05cb143081
--- /dev/null
+++ b/tests/compiler/duplicate-function-in-scope.json
@@ -0,0 +1,7 @@
+{
+  "asc_flags": [],
+  "stderr": [
+    "EOF",
+    "TS2300: Duplicate identifier 'inner'."
+  ]
+}
diff --git a/tests/compiler/duplicate-function-in-scope.ts b/tests/compiler/duplicate-function-in-scope.ts
new file mode 100644
index 0000000000..62c2e68a00
--- /dev/null
+++ b/tests/compiler/duplicate-function-in-scope.ts
@@ -0,0 +1,16 @@
+// Duplicate named function declarations in the same scope should
+// produce a diagnostic instead of crashing the compiler.
+
+export function test(): void {
+  function inner(): i32 {
+    let x: i32 = 0;
+    return x;
+  }
+  function inner(): i32 {
+    let x: i32 = 0;
+    return x + 1;
+  }
+  inner();
+}
+
+ERROR("EOF");