Double Identity by Margaret Peterson Haddix

Bethany Cole led a rather normal life until one night when her parents mysteriously dropped her off at an aunt's house without an explanation. Bethany had never met this aunt and wasn't sure she was really a relative of hers. When people in Aunt Myrlie's town see Bethany, they stare in shock. Bethany looks just like a girl who used to live there but died 20 years ago. Slowly, Bethany uncovers clues to her real identity and the reality that someone is after her.

Eleven by Patricia Reilly Giff

The day before his 11th birthday, Sam is snooping around trying to find his presents when he comes across a newspaper clipping about a missing 3-year old boy. He recognizes the boy as himself, but he can't read well enough to find out why he was missing. He has lived with Mack, his grandfather, for as long as he can remember, and Mack told him his parents had died. While working on a class assignment to build a castle for a medieval unit, Caroline, a new girl, helps him put together the pieces of his identity puzzle.