Made a duplicate artist tag

I created an artist tag marukado09 since the autotagger for pixiv said it didn't recognize the artist. However, on one of their recent uploads, when I tried to upload it here, I got a match to the older artist tag marui_kado.

What should I do in this scenario? Make an alias request? If so, which name should be the preferred one?

Yeah, you can make an alias request in topic #7713. Usually the most used name is the one you should pick as the tag name. In this case, marukado09 appears to be the stacc pixiv name, while marui kado is the display name in both their twitter and their pixiv, so that should be the tag name.