diff --git a/opt_einsum/parser.py b/opt_einsum/parser.py
index 1f3daf8..e390ec5 100644
--- a/opt_einsum/parser.py
+++ b/opt_einsum/parser.py
@@ -379,8 +379,10 @@ def parse_einsum_input(operands: Any, shapes: bool = False) -> Tuple[str, str, L
     else:
         input_subscripts, output_subscript = subscripts, find_output_str(subscripts)
 
-    # Make sure output subscripts are in the input
+    # Make sure output subscripts are unique and in the input
     for char in output_subscript:
+        if output_subscript.count(char) != 1:
+            raise ValueError("Output character '{}' appeared more than once in the output.".format(char))
         if char not in input_subscripts:
             raise ValueError("Output character '{}' did not appear in the input".format(char))
 
diff --git a/opt_einsum/tests/test_input.py b/opt_einsum/tests/test_input.py
index 37a6db9..17dc0ac 100644
--- a/opt_einsum/tests/test_input.py
+++ b/opt_einsum/tests/test_input.py
@@ -81,92 +81,92 @@ def test_type_errors():
         contract(views[0], [Ellipsis, dict()], [Ellipsis, "a"])
 
 
-def test_value_errors():
+@pytest.mark.parametrize("contract_fn", [contract, contract_path])
+def test_value_errors(contract_fn):
     with pytest.raises(ValueError):
-        contract("")
+        contract_fn("")
 
     # subscripts must be a string
     with pytest.raises(TypeError):
-        contract(0, 0)
+        contract_fn(0, 0)
 
     # invalid subscript character
     with pytest.raises(ValueError):
-        contract("i%...", [0, 0])
+        contract_fn("i%...", [0, 0])
     with pytest.raises(ValueError):
-        contract("...j$", [0, 0])
+        contract_fn("...j$", [0, 0])
     with pytest.raises(ValueError):
-        contract("i->&", [0, 0])
+        contract_fn("i->&", [0, 0])
 
     with pytest.raises(ValueError):
-        contract("")
+        contract_fn("")
     # number of operands must match count in subscripts string
     with pytest.raises(ValueError):
-        contract("", 0, 0)
+        contract_fn("", 0, 0)
     with pytest.raises(ValueError):
-        contract(",", 0, [0], [0])
+        contract_fn(",", 0, [0], [0])
     with pytest.raises(ValueError):
-        contract(",", [0])
+        contract_fn(",", [0])
 
     # can't have more subscripts than dimensions in the operand
     with pytest.raises(ValueError):
-        contract("i", 0)
+        contract_fn("i", 0)
     with pytest.raises(ValueError):
-        contract("ij", [0, 0])
+        contract_fn("ij", [0, 0])
     with pytest.raises(ValueError):
-        contract("...i", 0)
+        contract_fn("...i", 0)
     with pytest.raises(ValueError):
-        contract("i...j", [0, 0])
+        contract_fn("i...j", [0, 0])
     with pytest.raises(ValueError):
-        contract("i...", 0)
+        contract_fn("i...", 0)
     with pytest.raises(ValueError):
-        contract("ij...", [0, 0])
+        contract_fn("ij...", [0, 0])
 
     # invalid ellipsis
     with pytest.raises(ValueError):
-        contract("i..", [0, 0])
+        contract_fn("i..", [0, 0])
     with pytest.raises(ValueError):
-        contract(".i...", [0, 0])
+        contract_fn(".i...", [0, 0])
     with pytest.raises(ValueError):
-        contract("j->..j", [0, 0])
+        contract_fn("j->..j", [0, 0])
     with pytest.raises(ValueError):
-        contract("j->.j...", [0, 0])
+        contract_fn("j->.j...", [0, 0])
 
     # invalid subscript character
     with pytest.raises(ValueError):
-        contract("i%...", [0, 0])
+        contract_fn("i%...", [0, 0])
     with pytest.raises(ValueError):
-        contract("...j$", [0, 0])
+        contract_fn("...j$", [0, 0])
     with pytest.raises(ValueError):
-        contract("i->&", [0, 0])
+        contract_fn("i->&", [0, 0])
 
     # output subscripts must appear in input
     with pytest.raises(ValueError):
-        contract("i->ij", [0, 0])
+        contract_fn("i->ij", [0, 0])
 
     # output subscripts may only be specified once
     with pytest.raises(ValueError):
-        contract("ij->jij", [[0, 0], [0, 0]])
+        contract_fn("ij->jij", [[0, 0], [0, 0]])
 
     # dimensions much match when being collapsed
     with pytest.raises(ValueError):
-        contract("ii", np.arange(6).reshape(2, 3))
+        contract_fn("ii", np.arange(6).reshape(2, 3))
     with pytest.raises(ValueError):
-        contract("ii->i", np.arange(6).reshape(2, 3))
+        contract_fn("ii->i", np.arange(6).reshape(2, 3))
 
     # broadcasting to new dimensions must be enabled explicitly
     with pytest.raises(ValueError):
-        contract("i", np.arange(6).reshape(2, 3))
-    with pytest.raises(ValueError):
-        contract("i->i", [[0, 1], [0, 1]], out=np.arange(4).reshape(2, 2))
-
-
-def test_contract_inputs():
+        contract_fn("i", np.arange(6).reshape(2, 3))
+    if contract_fn is contract:
+        # contract_path does not have an `out` parameter
+        with pytest.raises(ValueError):
+            contract_fn("i->i", [[0, 1], [0, 1]], out=np.arange(4).reshape(2, 2))
 
     with pytest.raises(TypeError):
-        contract_path("i->i", [[0, 1], [0, 1]], bad_kwarg=True)
+        contract_fn("i->i", [[0, 1], [0, 1]], bad_kwarg=True)
 
     with pytest.raises(ValueError):
-        contract_path("i->i", [[0, 1], [0, 1]], memory_limit=-1)
+        contract_fn("i->i", [[0, 1], [0, 1]], memory_limit=-1)
 
 
 @pytest.mark.parametrize(