In this regard, why does Spotify have duplicate albums?
Saved (and/or downloaded) albums can be duplicated when a silent reupload happens which can result in extra space being used, a reduction listening experience due to repeat tracks being heard and the loss of saved music if not managed properly among such other issues.
Similarly, does Spotify download songs twice? My understanding is that each and every song is counted in terms of your downloads. So if you have 500 songs and 40 are duplicates or even triplicates, your number of downloaded songs will be 500. If you have two songs by the same artist but on different albums, that would be two unique songs.
Similarly, you may ask, how do I remove duplicates from a playlist?
Click on the Music category (top left of iTunes) then choose the File Menu and select Show Duplicates. From there you can select all and delete (Command Delete) all of the duplicates, and move the files to the trash when prompted.
How do I stop duplicates on Spotify?
Click on one of the tabs located at the top to list the songs in order of artist or album. Any duplicates should stack together, although different albums sometimes have the same songs. Go through the list and delete any that you can find. It shouldn't take that long, and it'll give you more space for new songs.