But when duplicates are stripped, it should never become larger than all songs on the player. I thought that if I enqueue things with after a search, that other occurences of that song are implicitly taken out of the existing playlist.

In fact, a regular jukebox typically doesn't allows the same song to appear more than once in the upcoming playlist. Probably to prevent someone (who is leaving a bar) from selecting the 20 minute version of freebird several times in a row and ruin the evening for everyone else.