Also, are there any rules about reforming? Must they touch? Does the dupe simply dissapear into mist and the dupling is 'whole' again? What happens if you forcecage a dupe, and prevent it from returning to the prime?

I think you should have the division of items between the two happen as the dupe is formed. Like, a dupling wearing a ring of invisibility, a gauntlet of rust, a +2 longbow and wings of flying, the dupling could could decide the dupe gets the ring and the gauntlet to go make a mess of the armory, while the prime keeps the wings and bow to go distract the guards. They both get illusionary/mundane copies of the gear the other took.

While randomeness is a good thing, this seems like something that would encourage planning. And you can't plan it if you expect the dupe to come back in 6 rounds, but it turns out to have a time limit of 4. I'd say make it 3 or 4 tound plus your wisdom mod. That would represent the stress of sharing two completely different perspectives at once. Make the feat make it the same number, just in minutes, instead.

For the casting issue: perhaps it could be that the stress of manipulating magic is too great to do in two places at once. So, only once copy could do magic in each round, but they are both capable of it.

Can a dupling flank with itself?

On an unrelated note, maybe this idea would be better served as a template. Like, there's a spell or ritual that gives it to them, and then any creature can divide.