@graemeandrew great questions again! The tag function was added just before our developer left, and we thought it was better to have something than hold it back until we have someone in post to finish it perfectly (as these things can sometimes take longer than you'd think). What does the pop-up message say?
The short answer is, we'll be able to sort through the duplicate tags later, so it's still worth adding them.
'Not a playbill' is for blank sheets or the occasional other sheet that's been digitised as part of the volume. If there's not a 'not a performance' option then we should add it! After talking to scholars about it, we haven't worried too much if performances that aren't strictly plays are included, as we'll probably do a quick classification task at some point to sort them out.