The split function not working properly and duplicating the item to more than you originally had is most likely a result of playing on a cracked version. The real version of the game doesn't do this.

If you are playing on a legitimate version, purchased from the App Store, Play Store, or Amazon and you are still having trouble with splitting stacks, please let me know and we can troubleshoot.
Beyond that, I’m going to move this out of the bug section as it hasn’t been able to be replicated, and mark it resolved.