Warn user they are already Scheduled to work before accepting a shift swap
If the proposed covering person is already scheduled to work (or work time is overlapped) then a warning should be displayed, or person omitted from the list of optional cover/swap. After all you can't cover somebody whilst doing your own job too.
This would be a great feature to have. Or if the request was not sent to those already working at that time.
I've only been playing around for a few hours, but I don't see any warning of being scheduled in two places at once. Adding in that kind of warning would be great for writing the initial schedule.