*if youve read city of glass* I just finished COG and why would Tessa look familiar to clary?

Well, if she's immortal, which we're all pretty sure she is, then she wouldn't be a ghost, and she wouldn't have to go into the future. She seems to be friends with Magnus, and so it's really not surprising that she'd be around him. As for why she might look familiar to Clary, how about this: you know how Clary's memories were erased? Maybe she had seen her before, just walking down the street or something, but the memory was erased. Upon seeing her again, the memory would surface.