CI Flake `test_gossip_force_broadcast_channel_msgs`

It appears sometimes two channels are announced instead of the expected 1. Either two channel announcements is fine or the duplicate announcement is a sign of some underlying bug.

([CI Log](https://github.com/ElementsProject/lightning/actions/runs/28471872223/job/84389393698))

```
def test_gossip_force_broadcast_channel_msgs(node_factory, bitcoind):
""" Send our own channel_update, node_announcement or channel_announcement to existing peers, even if they say they're not interested.
"""
l1, l2 = node_factory.get_nodes(2)

# One block away from being announced.
node_factory.join_nodes([l1, l2], wait_for_announce=False)
bitcoind.generate_block(4)

# It will send timestamp_filter. It should also send a channel_announcement, a
# channel_update, and a node_announcement, even though we said no gossip.
# It may or may not send:
# query_short_channel_ids, query_channel_range, a ping (which we gossipwith replies to)
process = subprocess.Popen(['devtools/gossipwith',
'--no-gossip',
'--hex',
'--network={}'.format(TEST_NETWORK),
'--max-messages={}'.format(7),
'--handle-pings',
'--timeout-after=30',
'{}@localhost:{}'.format(l1.info['id'], l1.port)],
stdout=subprocess.PIPE, stderr=subprocess.PIPE)
# Now do the final announcement
bitcoind.generate_block(1)

stdout, stderr = process.communicate(timeout=30 + TIMEOUT)
assert process.returncode == 0, f"Exit failed: output = {stderr}"

lines = stdout.decode('utf-8').splitlines()
types = {'0100': 'channel_announce',
'0102': 'channel_update',
'0101': 'node_announce',
'0105': 'query_short_channel_ids',
'0107': 'query_channel_range',
'0109': 'gossip_filter',
'0012': 'ping'}
tally = {key: 0 for key in types.values()}
for l in lines:
tally[types[l[0:4]]] += 1

# Make sure the noise is within reasonable bounds
assert tally['query_short_channel_ids'] <= 1
assert tally['query_channel_range'] <= 1
assert tally['ping'] <= 5
assert tally['gossip_filter'] >= 1
del tally['query_short_channel_ids']
del tally['query_channel_range']
del tally['ping']
del tally['gossip_filter']
> assert tally == {'channel_announce': 1,
'channel_update': 1,
'node_announce': 1}
E AssertionError: assert {'channel_announce': 2, 'channel_update': 1, 'node_announce': 1} == {'channel_announce': 1, 'channel_update': 1, 'node_announce': 1}
E
E Common items:
E {'channel_update': 1, 'node_announce': 1}
E Differing items:
E {'channel_announce': 2} != {'channel_announce': 1}
E
E Full diff:
E {
E - 'channel_announce': 1,
E ? ^
E + 'channel_announce': 2,
E ? ^
E 'channel_update': 1,
E 'node_announce': 1,
E }

tests/test_gossip.py:2408: AssertionError
```

Start with tests/test_gossip.py:2408, reproduce the failure using devtools/gossipwith, and review the linked CI log. Determine whether two channel announcements are valid or indicate a duplicate in the gossip behavior. Done means the behavior is resolved and the test reliably reflects the intended announcement count.

Written by the indexing model from the issue text.