Once a match has already been synced from the master to other tablets, you can’t make any additional changes as they are locked into “scoring mode.”
When you added a 5th stage to all the tablets, the master reads it as multiple individual stages needing to be adding since they weren’t on your master when you synced in the first place. The only way to have all the scores in one “5th stage,” you’ll need to manually enter them and then delete the second and third 5th stage.
This is not the easiest option, and definitely not the only way.
Another option is to use “Reassign to another” stage from the scores Review screen for each shooter on the duplicated stages, but that is not the most efficient way.
@Rich_Simmons if you have Android device with PractiScore app installed, sync all tablets to it, then open the “Build Stages” screen, then tap and hold on these duplicated stages, then you will see the “Merge” option at the top. More on that at How to merge duplicated stages