If they're meta type units, I'll keep them. Otherwise BB fodder. Right now the only units I have dupes of are Darvanshel, Zelnite, and Lilly Matah.
Only other current units I'd keep a dupe of are probably Arus, Ulkina, Tridon, Elza, Kuda (only because I like him), and Mifune (for the future)...