1 files changed, 4 insertions, 1 deletions
diff --git a/check.hs b/check.hs
index d2a7c92..1a0a24c 100644
--- a/check.hs
+++ b/check.hs
@@ -91,7 +91,10 @@ typeCheck (Program decls) = Program <$> mapM (goD topLevelNames) decls
 
     goS :: Type  -- function return type
         -> Map.Map Name Type -> Statement -> Error (Map.Map Name Type, Statement)
-    goS _ names st@(StVarDeclaration t n Nothing) = return (Map.insert n t names, st)
+    goS _ names st@(StVarDeclaration t n Nothing) = do
+        maybe (return (Map.insert n t names, st))
+              (const $ Left $ "Duplicate variable '" ++ n ++ "'")
+              (Map.lookup n names)
     goS frt names (StVarDeclaration t n (Just e)) = do
         (newnames, _) <- goS frt names (StVarDeclaration t n Nothing)
         (_, StAssignment _ newe) <- goS frt newnames (StAssignment n e)