Hi @Graham Clark !

Unfortunately, there is no quick solution for searching for a specific song on all playlists.
You can sort the songs on each playlist alphabetically and then check on each playlist if that song is there, but that requires a lot of time and effort (for this operation I recommend the Desktop Deezer app)
Or you can create a new playlist and transfer all the songs from your 10 playlists to it, then it will be possible to better control the songs that are double .
Only now all the songs will be on one playlist, I don't know how acceptable that will be to you

Hi. I have the deezer app in android. I used to be able to search for a particular track in my playlist but now  I can't. Is there a reason for this? 

To search for a song in a playlist you can use the search option for that playlist, but you have to open it first. Then you should be able to see the search icon, at least in the iOS app, web player and desktop app, so I guess the Android app also has it.

To @Graham Clark: Usually there is an automatic check in Deezer if you already have added a song to a playlist, but it does not check all  playlists then, only the one you want to add the song to. So you could e.g. add all your playlist songs to your favorite songs or to a new playlist if they are less than 2k tracks and either use the search option with it or try to add a new song for checking if it is already included there.