I have 3 Crystal unicorns. I haven't had to consider selling as they don't even fill the 2 grand gem groves I bought when they were on sale.
I have sold all other duplicates, except ones I have in storage. Oh, I have kept the baby Christmas poodle, which I think is much cuter than the adult one. When push comes to shove I don't know which one I will sell.... Lately, if there's room I'll save a plantlers or other common or rare that might be used for feeding goals. But they get tossed if need be.