I have a friend who I will call Sammy in my friend group and she always crushes on and goes after whoever my other friends like. She will secretly go after them and tell everyone else she likes them but not tell the friend who also likes them. And if Sammy is confronted she will deny it. Also if the person who liked the guy to begin with loses interest or gives up hope of being with him Sammy will drop him too. I really want to know if there is a word or condition for this.
If you'll be pleased with a scientific term for this (meaning, not something you'd readily say to someone unless you cared to explain it):
As the Scientific American article opens, it describes this situation perfectly:
I'm sure we've all seen it. Kid A is playing with a toy, and the next thing you know, Kid B wants it, too. Even when there are other toys around, Kid B is no longer satisfied. He wants what Kid A has got, a phenomenon called "mimetic desire", in which one person's desire mimics that of another. Mimetic desire isn't limited to children, we've all wanted that new computer once we've seen what someone else has. The selling of luxury items has depended upon this for ages: did you really want ice cream? Or did it look a lot better once everyone else had some?
I'd really recommend reading the full article if you care to get more informed about the way your own brain works, because we're all susceptible to this impulse.
I think it is simply the verb "to covet".