Remove redundant name check when adding QuantumCircuit registers.

[davidfcohen]

Description

QuantumCircuit wraps RegisterData on the Rust side. RegisterData::add_register returns the RegisterAlreadyExists when the name is in use. This error is properly raised on the Python side without manual checking.

The error is handled on the Rust side; thus, the check is redundant in the Python code. I removed the redundant check in qiskit/circuit/quantumcircuit.py.

Generally, the overall time complexity of checking whether a given register name exists was reduced from O(n) to O(1).

Fix #7133

Performance

Registers Added	Before (ms)	After (ms)
100	7	5
1000	478	22
10000	45211	216

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description:
• I used the following tool to generate or modify code:

[CLAassistant]

All committers have signed the CLA.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[raynelfss]

This is a good start. Thank you for being so steadfast.

The fix itself is good. However, there are some nuances I pointed out during the review here.

You may also want to drop the "[WIP]" from PR title.

[raynelfss]

This is about ready to merge. I have a couple formatting suggestions for your release note here.

[raynelfss]

This LGTM, thank you the quick turnaround.

[Cryoris]

@davidfcohen quick follow up note: it seems the error triggered now has a word repetition/typo, could we fix that while we're at it?

from qiskit import QuantumCircuit, QuantumRegister

qr1 = QuantumRegister(1, "a")
qr2 = QuantumRegister(1, "a")

qc = QuantumCircuit(qr1, qr2)

gives now

qiskit.circuit.exceptions.CircuitError: 'register name register name "a" already exists already exists'

(the "already exists" is duplicated)