frankly:
@Mulberry_owl, this is the best I could find for the instrumental background music: https://www.youtube.com/watch?v=hTQsqldA_ts&list=PLxuaaN221HuBHKsZSmfNl4__G4ONv4E4t. Note that you may hear some background noises as the person took the music from the drama and tried to remove the talking/sounds/etc. Not sure if they have released the instrumental OST yet, but I'd like to know as well...
Thank you for answering!
I'm aware of this playlist's existence, but still, thank you for posting it here, making it available for everyone.
To my understanding, the instrumental OST / BGM for "The Double" is the following:
https://www.wlyxmusic.net/thread-21200-1-1.html
http://www2.kugou.kugou.com/yueku/v9/album/single/96005343-91.html
A total of 18 songs with lyrics (the ones @Peachey Blossom very kindly posted, along with their instrumental versions) + 99 background music tracks = 117 unique tracks for the drama.
I loved the background ost and would like to be able to listen to it properly (without dialogue), but unfortunately I don't speak chinese and have no idea how to download these tracks, so I decided to post my question here in case someone who has knowledge and access sees it and has the time to help me.