Ideally, this is in answer to a YKTTW post that actually asked "Administrivia/DoWeHaveThisOne". Otherwise, it would be very embarrassing for the original poster.
Typically a response takes the form of a single link to the trope page, with no explanation. There may even be simultaneous posting by multiple experienced tropers. Again, ''ideally'', this should be sufficient to end the conversation. Yet people can and will continue to post afterwards, particularly if the thread has already gained a large number of posts that newcomers don't bother to read through before replying. '''BoldInflation''' was proposed as a potential cure for this epidemic of illiteracy not long ago.
Occasionally, non-ideal situations happen. Sometimes it's not immediately clear that they're the same trope and it needs to be explained. Other times, it could be the person blowing the "we have this" whistle who has misunderstood the trope involved. This is especially prone to happen if its name is very similar to an existing trope despite not being related. Pointing out at the bottom of the description that it's not to be confused with the similarly-named trope will normally prevent this.
The question of whether proper wiki etiquette would be "launching" the YKTTW to Administrivia/DiscardedYKTTW, to the pre-existing trope page, or waiting for the OP to return and do so themselves, remains unresolved.