Check the existence of AncillaQubits before adding

qiskit/circuit/quantumcircuit.py

@@ -1352,7 +1352,9 @@ def add_register(self, *regs: Union[Register, int, Sequence[Bit]]) -> None:
                 raise CircuitError('register name "%s" already exists' % register.name)
 
             if isinstance(register, AncillaRegister):
-                self._ancillas.extend(register)
+                for bit in register:
+                    if bit not in self._qubit_indices:
+                        self._ancillas.append(bit)
 
             if isinstance(register, QuantumRegister):
                 self.qregs.append(register)

releasenotes/notes/fix-adding-ancilla-register-without-checking-abe367dab5a63dbb.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    :meth:`.QuantumCircuit.add_register` will no longer cause duplicate
+    :class:`.AncillaQubit` references in a circuit when given an
+    :class:`.AncillaRegister` whose bits are already present.

test/python/circuit/test_circuit_operations.py

@@ -22,6 +22,7 @@
 from qiskit.circuit.classicalregister import Clbit
 from qiskit.circuit.exceptions import CircuitError
 from qiskit.circuit.quantumcircuit import BitLocations
+from qiskit.circuit.quantumregister import AncillaQubit, AncillaRegister, Qubit
 from qiskit.test import QiskitTestCase
 from qiskit.circuit.library.standard_gates import SGate
 from qiskit.quantum_info import Operator
@@ -948,6 +949,41 @@ def test_compare_a_circuit_with_none(self):
 
         self.assertFalse(qc1 == qc2)
 
+    def test_overlapped_add_bits_and_add_register(self):
+        """Test add registers whose bits have already been added by add_bits."""
+        qc = QuantumCircuit()
+        for bit_type, reg_type in (
+            [Qubit, QuantumRegister],
+            [Clbit, ClassicalRegister],
+            [AncillaQubit, AncillaRegister],
+        ):
+            bits = [bit_type() for _ in range(10)]
+            reg = reg_type(bits=bits)
+            qc.add_bits(bits)
+            qc.add_register(reg)
+
+        self.assertEqual(qc.num_qubits, 20)
+        self.assertEqual(qc.num_clbits, 10)
+        self.assertEqual(qc.num_ancillas, 10)
+
+    def test_overlapped_add_register_and_add_register(self):
+        """Test add registers whose bits have already been added by add_register."""
+        qc = QuantumCircuit()
+        for bit_type, reg_type in (
+            [Qubit, QuantumRegister],
+            [Clbit, ClassicalRegister],
+            [AncillaQubit, AncillaRegister],
+        ):
+            bits = [bit_type() for _ in range(10)]
+            reg1 = reg_type(bits=bits)
+            reg2 = reg_type(bits=bits)
+            qc.add_register(reg1)
+            qc.add_register(reg2)
+
+        self.assertEqual(qc.num_qubits, 20)
+        self.assertEqual(qc.num_clbits, 10)
+        self.assertEqual(qc.num_ancillas, 10)
+
     def test_deprecated_measure_function(self):
         """Test that the deprecated version of the loose 'measure' function works correctly."""
         from qiskit.circuit.measure import measure