I prefer editing my post after I saw someone replied before me (if I need to edit indeed).
When we posted a reply, it already displays our post on the thread and unless we are in a hurry to close the window just after hitting Post Reply button, we can already see if someone posted before us.
I would be really annoyed with such a warning, and would WDZ to implement an option for it, and things will get more difficult and more annoying
There is also a quick deletion button, and I sometimes use it when someone posted the same reply just before me.
By the way, john-t's this suggestion reminded me one of my crap suggestion for the forum, whatever
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