Just hit the repost button again. Presumably it stays visible until the feed refreshes, but seeing as Instagram seems to refresh every 30 seconds with another dozen ads and posts from random strangers, it will disappear pretty quickly.

And I’ve just found the “your reposts” tab, so you can do it even if you lose the original post in the churn. Jeez, though - would a “confirm” setting be so bad?

With every update, it gets less and less pleasant to use.