Can I set the same relation in a field multiple times? My customer wants a videoplaylist for users.

Idealy I'd like to have a assets relation field for the playlist and relate all the videos in said playlist.

The problem is if a user wants the same video several times in the playlist. 
( like in a loop ).

As far as I know it's only possible to relate an entry/asset once in the same relation field.

The solution for me would be to add a matrix-block for each video in the playlist, but since there are thousands of playlists the db grows way more than expected.

Without matrix field: around 30MB
With matrix blocks: around 200MB.

The site slowed down quite a bit and the searchindex table grew to about 80MB.

Is there another way to implement this or a ideally a way to relate an entry several time in one field?
You're right, this can only be accomplished with a Matrix field.

That said, you could probably accomplish what you need via a custom field type. You'd need to create a custom plugin (obviously), but you could probably build something that simply stores an array of asset IDs:

[12,33,42,10,10,10]


I won't diver deeper into what the mechanics of that plugin &amp; field type should be... that's a bit beyond the scope of this question.