Detect duplicates in QuantumCircuit.compose

Previously, `QuantumCircuit.compose` would silently attempt to compose
multiple qubits onto the same state, which would fail in weird ways if
any multi-qubit instructions were acting on these collapsed qubits.

qiskit/circuit/quantumcircuit.py

@@ -1008,6 +1008,10 @@ def compose(
                     f"Number of items in qubits parameter ({len(mapped_qubits)}) does not"
                     f" match number of qubits in the circuit ({len(other.qubits)})."
                 )
+            if len(set(mapped_qubits)) != len(mapped_qubits):
+                raise CircuitError(
+                    f"Duplicate qubits referenced in 'qubits' parameter: '{mapped_qubits}'"
+                )
             edge_map.update(zip(other.qubits, mapped_qubits))
 
         if clbits is None:
@@ -1019,6 +1023,10 @@ def compose(
                     f"Number of items in clbits parameter ({len(mapped_clbits)}) does not"
                     f" match number of clbits in the circuit ({len(other.clbits)})."
                 )
+            if len(set(mapped_clbits)) != len(mapped_clbits):
+                raise CircuitError(
+                    f"Duplicate clbits referenced in 'clbits' parameter: '{mapped_clbits}'"
+                )
             edge_map.update(zip(other.clbits, dest.cbit_argument_conversion(clbits)))
 
         variable_mapper = _classical_resource_map.VariableMapper(

releasenotes/notes/fix-circuit-compose-duplicate-59de6c1c51f58e54.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    :meth:`.QuantumCircuit.compose` will now correctly raise a :exc:`.CircuitError` when there are
+    duplicates in the ``qubits`` or ``clbits`` arguments.

test/python/circuit/test_compose.py

@@ -31,6 +31,7 @@
     Instruction,
     CASE_DEFAULT,
     SwitchCaseOp,
+    CircuitError,
 )
 from qiskit.circuit.library import HGate, RZGate, CXGate, CCXGate, TwoLocal
 from qiskit.circuit.classical import expr
@@ -880,6 +881,16 @@ def test_expr_target_is_mapped(self):
 
         self.assertEqual(dest, expected)
 
+    def test_rejects_duplicate_bits(self):
+        """Test that compose rejects duplicates in either qubits or clbits."""
+        base = QuantumCircuit(5, 5)
+
+        attempt = QuantumCircuit(2, 2)
+        with self.assertRaisesRegex(CircuitError, "Duplicate qubits"):
+            base.compose(attempt, [1, 1], [0, 1])
+        with self.assertRaisesRegex(CircuitError, "Duplicate clbits"):
+            base.compose(attempt, [0, 1], [1, 1])
+
 
 if __name__ == "__main__":
     unittest.main()