This question is dupe to this question. (I was the first user to flag my question as dupe.) Mods decided to mark it a dupe to another question, that's none of my business. Basically reading this question would help me solve my problem.
But how to reach it? My question has a totally different title, therefore it wasn't shown in the suggested list. I can't just go brainstorming, thinking of different titles until I nail it :)
It didn't even popup on google, and I am an avid googler. Moreover if you use this error message
Problem with MergeList /var/lib/apt/lists/repository.spotify.com_dists_stable_non-free_i18n_Translation-en%5fUS
And you just put it in the askubuntu search bar, it would return this list. Both Google and SE search engine may contain links to questions similar to mine (like the one you marked my question a dupe to); but I was specifically asking about spotify, I didn't want to go try random solutions, and as it turns out, Spotify repo is bad, just had to remove it, no further answers or explanations are needed.
I like to know just enough to fix a specific problem, otherwise I would have used arch reading endless docs, learning solutions to problems I may never have!
I never reached the duplicate that I was looking for, although I used the most specific method, searching for the error itself rather than random keywords. Who's to blame me or SE? And what can we do about it?
I decided to ask this question because many of my questions are marked duped on many SE sites, though I really tried to find those dupes but couldn't. Sometimes the user use a different way to ask a question, making it impossible to find it, at least for me anyhow.