Library limit reached; how to identify songs already in playlists?

[Nearlynubile]

Plan

Premium

Device

Applies to all devices 

Operating System

All OSes. 

My Question or Issue
I'm one of the power users I suppose -- I love 'Discover Weekly'and the 'Discover' to find music. When I find good songs, I add them to Playlists. If I'm adding a song to the same playlist twice, Spotify shows me the duplicate warning. But no way to find if I'm adding a song to Playlist 2 that I already have in Playlist 1. To enableme to identify that a given song is in *any of the playlists* I've been using the "Library" (the plus sign) function. This turns into a green tick when I add it. Other than this, I have no use for the library or the 'daily mixes' etc, because I sort and shuffle my music according to my tastes. 

The issue now is this: my Library is apparently "full". Searching for help I find this is some random 'feature' from Spotify that caps this at 9999 songs or some silly thing like that. I'm totally fine with Library being as useless as it is especially with that cap.

All I'm seeking is:
To know that a song I'm listening to is in *one of my* playlists. As in: to somehow visually identify that I've added it to my self-managed repertoire of music according to my categories of taste, not by genre etc. Instead of adding it to all playlists one by one and seeing if the duplicate alert pops up...which would be a moronic way to do this basic music management task. 

How can I do this? Many thanks for any pointers. 

[rhettdean]

This is time consuming, but the only way I have found to do this is to:

1. Copy all your songs in all your playlists into one “Master Playlist”. The fastest way to do this is from the desktop app where you can select all the songs in a playlist at once and click and drag to the Master Playlist. 

2. After that heavy lifting is done, you can then open the Master Playlist and sort by song title. Then scroll through and look for which duplicates you have. 

3. Once you find a duplicate, you can then go back into the playlist that song lives in and delete out the duplicate version.