diff --git a/artiq/py2llvm/typing.py b/artiq/py2llvm/typing.py
index 5d2b140a9..29981389a 100644
--- a/artiq/py2llvm/typing.py
+++ b/artiq/py2llvm/typing.py
@@ -71,6 +71,12 @@ class LocalExtractor(algorithm.Visitor):
             self.typing_env[name] = types.TVar()
 
     def visit_arg(self, node):
+        if node.arg in self.params:
+            diag = diagnostic.Diagnostic("error",
+                "duplicate parameter '{name}'", {"name": node.arg},
+                node.loc)
+            self.engine.process(diag)
+            return
         self._assignable(node.arg)
         self.params.add(node.arg)
 
diff --git a/lit-test/py2llvm/typing/error_nonlocal_global.py b/lit-test/py2llvm/typing/error_locals.py
similarity index 89%
rename from lit-test/py2llvm/typing/error_nonlocal_global.py
rename to lit-test/py2llvm/typing/error_locals.py
index 1242f56db..7d07d699b 100644
--- a/lit-test/py2llvm/typing/error_nonlocal_global.py
+++ b/lit-test/py2llvm/typing/error_locals.py
@@ -20,6 +20,10 @@ def d(x):
     # CHECK-L: ${LINE:+1}: error: name 'x' cannot be a parameter and global simultaneously
     global x
 
-def d(x):
+def e(x):
     # CHECK-L: ${LINE:+1}: error: name 'x' cannot be a parameter and nonlocal simultaneously
     nonlocal x
+
+# CHECK-L: ${LINE:+1}: error: duplicate parameter 'x'
+def f(x, x):
+    pass