Buffy is dating William, his twin brother Spike shows up and a love triangle ensues

Hi! I'm looking for a fic where Buffy is dating William but his twin brother Spike shows up and confuses things between them. William ends up trying to dress/look more like Spike and I think at one point she has sleeps with one thinking he is the other. Thanks

Are you thinking of this one?

https://www.spikeluver.com/Forum/index.php?action=vthread&forum=1&topic=3035

There are other twin fics out there but if this is the one you are seeking there's no need to search further (it's only a couple threads down).

I think it's this one.

Mixed Emotions by Ashlee

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=4802&warning=NC-17

Summary: Buffy is in a committed relationship with William Lawson. Everything is going great - until his twin brother Spike comes to town.

It's complete.

I think it was Mixed Emotions, thanks!