Many many years ago i watched a film on the tv about a guy who clones himself. The clone is a perfect replica apart from occasionally one of his eyes turns totally white. Towards the end of the film both the guy and clone have a fight. One killing the other with a big plank of wood (if i remember correctly). The film ends with the viewer not knowing who won the fight.....the guy or his clone!
i remember a film about a man with an evil boil on his shoulder which comes to life and gets bigger and bigger and eventually takes over the man himself and lives his life. is that what you mean?
The film with the boil is the excellent Richard E. Grant movie 'How to get a Head in Advertising'. He starts off nice but the the evil boil head that is ruthless soon takes over - but the closing shot of that fil is him standing on a hillside.