As described in support request Massive duplication of playlist entries suddenly appears - #6 by Brian_Marick, the user interface makes it too easy to copy one playlist into another when you actually only wanted to add one or a few tracks. Because (for example) deleting duplicates has to be done one track at a time, this can be really hard to fix. Because I clicked the wrong button, I’ve spent 3 hours today fixing a playlist. I’m up to “J”.
I think a “do you mean to add 5023 tracks?” confirmation dialog would be appropriate.