diff --git a/toy-impl/parse_expr.py b/toy-impl/parse_expr.py
index e699ca1..98e34d8 100644
--- a/toy-impl/parse_expr.py
+++ b/toy-impl/parse_expr.py
@@ -208,6 +208,8 @@ def parse_if_expr(ctx: Context,
 
 def parse_binding(name, ty):
     if isinstance(name, ast.Name):
+        if name.id == '_':
+            return {}
         return {name.id: ty}
     elif isinstance(name, ast.Tuple):
         if not isinstance(ty, TupleType):
@@ -216,7 +218,11 @@ def parse_binding(name, ty):
             raise CustomError(f'pattern matching length mismatch')
         result = {}
         for x, y in zip(name.elts, ty.params):
+            binding = parse_binding(x, y)
+            expected = len(result) + len(binding)
             result |= parse_binding(x, y)
+            if len(result) != expected:
+                raise CustomError('variable name clash')
         return result
     else:
         raise CustomError(f'binding to {name} is not supported')
diff --git a/toy-impl/test_expr.py b/toy-impl/test_expr.py
index 102f928..d2c5c4a 100644
--- a/toy-impl/test_expr.py
+++ b/toy-impl/test_expr.py
@@ -83,6 +83,8 @@ test_expr('[x for x in [1, 2, 3]]', {})
 test_expr('[1 for x in [1, 2, 3] if x > 2]', {})
 test_expr('[a + a for x in [1, 2, 3] if x > 2]', {'a': I})
 test_expr('[a for a, b in [(1, 2), (2, 3), (3, 4)] if b > 2]', {})
+test_expr('[a for a, a in [(1, 2), (2, 3), (3, 4)] if b > 2]', {})
+test_expr('[1 for _, _ in [(1, 2), (2, 3), (3, 4)]]', {})
 test_expr('[b for a, b in [(1, (2, 3)), (2, (3, 4)), (3, (4, 5))] if b > 2]', {})
 test_expr('[b for a, b in [(1, (2, 3)), (2, (3, 4)), (3, (4, 5))]]', {})
 test_expr('[b for a, (b, c) in [(1, (2, 3)), (2, (3, 4)), (3, (4, 5))] if b > 2]', {})