diff --git a/src/compiler.ts b/src/compiler.ts
index 36f09ab8aa..759eb32c0b 100644
--- a/src/compiler.ts
+++ b/src/compiler.ts
@@ -1574,7 +1574,8 @@ export class Compiler extends DiagnosticEmitter {
     /** Force compilation of stdlib alternative if a builtin. */
     forceStdAlternative: bool = false
   ): bool {
-    if (instance.is(CommonFlags.Compiled)) return !instance.is(CommonFlags.Errored);
+    if (instance.is(CommonFlags.Errored)) return false;
+    if (instance.is(CommonFlags.Compiled)) return true;
 
     if (!forceStdAlternative) {
       if (instance.hasDecorator(DecoratorFlags.Builtin)) return true;
diff --git a/src/program.ts b/src/program.ts
index 8bcf1bfa19..e54826b19d 100644
--- a/src/program.ts
+++ b/src/program.ts
@@ -3850,7 +3850,15 @@ export class Function extends TypedElement {
         flow.setLocalFlag(local.index, LocalFlags.Initialized);
       }
     }
-    registerConcreteElement(program, this);
+    if (program.instancesByName.has(this.internalName)) {
+      program.error(
+        DiagnosticCode.Duplicate_function_implementation,
+        prototype.declaration.name.range
+      );
+      this.set(CommonFlags.Errored);
+    } else {
+      registerConcreteElement(program, this);
+    }
   }
 
   /** Gets the types of additional locals that are not parameters. */
diff --git a/tests/compiler/duplicate-function-in-scope.json b/tests/compiler/duplicate-function-in-scope.json
new file mode 100644
index 0000000000..be8607a6b1
--- /dev/null
+++ b/tests/compiler/duplicate-function-in-scope.json
@@ -0,0 +1,8 @@
+{
+  "asc_flags": [],
+  "stderr": [
+    "EOF",
+    "TS2300: Duplicate identifier 'inner'.",
+    "TS2393: Duplicate function implementation."
+  ]
+}
diff --git a/tests/compiler/duplicate-function-in-scope.ts b/tests/compiler/duplicate-function-in-scope.ts
new file mode 100644
index 0000000000..7d91e2abfc
--- /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 testMixed1(): void {
+  const inner = function (): void {};
+  function inner(): void {}
+  inner();
+}
+
+export function test(): void {
+  function inner(): void {}
+  function inner(): void {}
+  inner();
+}
+
+ERROR("EOF");