Currently, when Plastic sees a local file older than the one on the repo, it will be impossible to checkin. Moreover, it will tell you only for the first file it finds, so you have to try checkin again and fix each file individually as long as the error pops up.

Solution : Show a list of all the conflicts, and for each file (why not also have a button to process all..), let the user choose which file he wants to keep.
- Keep local file : checkin the local file ; overriding plastic warning.
- Keep repo file : undo change to local file. Plastic should warn you with confirmation popup "you are about to loose these files : [...]" when proceeding.