Fix a little thinko in sending events to secondary listeners. (#71997)

If `unique` is true, I was redoing the uniqueness check for the
secondary listeners, which is racy since a "duplicate" event could come
in between deciding to send the event to the main listener and checking
for the shadow listener, and then the two streams would get out of sync.

There's not a good way to write a direct test for this, but the
test_shadow_listeners test has been flakey and this is the only racy
part of that system I can find. So the test would be that that
test_shadow_listeners becomes not flakey.