I love Lois Duncan and thought that Stranger With My Face was fantastic. I really thought I was seeing myself everywhere after that. I'd encourage you to try it out again and to work through whatever slow parts you find.
I haven't read it for a few years, but I remember a bit of it.
The main girl, Laurie, doesn't look anything like her family and thinks it's genetics. Later on, she discovers she's been adopted and she was a twin. She has no clue who or where her other half is.
Her friends and family tell Laurie they've seen her doing weird things, but Laurie knows she definitely has not been doing the things they say. Somehow, Laurie discovers it's her twin that's been doing the weird things, like going really far into the ocean.
The twin is not there in person but by sending her soul out of her body. She teaches Laurie how to do it and Laurie begins to visit her twin and everything. It's not a good way for her to be because she's, obviously, not there, and people could think she's dead, etc., etc.
One day, Laurie leaves her body to visit her twin, but her twin is not where she's supposed to be, in an orphanage or something. Then Laurie discovers that she can't get back into her body.
Laurie's twin took over Laurie's body.
The twin starts to mess with her family and her life. Eventually, Laurie figures out that she has to use this sapphire necklace and do something or other that will destroy the twin from her body. The details are fuzzy, but I believe Laurie had to inhabit the twin's body.
In the end, Laurie gets her body back and fixes her life.
Obviously, it's not a great summary, but I got some of the finer points. I'm worried that I mixed up some details with other Duncan novels, but I'm pretty sure it went down in this fashion.
Again, it's a great book, I highly recommend it.