Brothers in Arms, by Lois McMaster Bujold
Part of the Vorkosigan Saga by Lois McMaster Bujold...
I really liked this one. It completely turns Miles' whole identity as being "one of a kind" on its head. There's a group (mostly the guy in charge of the group) that wants revenge on Miles' father the Count. So they clone Miles intending to replace him and cause all kinds of chaos when the clone gets back to Barrayar since he'll have all kinds of access, not to mention being third in line for the throne.
Of course since Miles' actual genetics aren't screwed up the clone has had to go through all kinds of pain since his creation to make him physically resemble Miles since genetically he is "normal" and therefore a clone would be "normal". I felt so bad for him. His entire existence has been pain and torture to satisfy the twisted desires of one man. It's no wonder he has some serious mental problems! There's just no way for him to trust anyone, even though we know he can and we know that Miles' and his family would help him and take care of him.
It isn't any treat for Miles either. He suddenly has a sibling, something he's always wanted, but said sibling has been practically bred to hate him and his family.
And there's that whole plot to destroy the Imperium that has to be stopped too...