Quote Originally Posted by lololink View Post
Goku/Djidane, sent to wreak havoc on earth, lost his memories, Vegeta/Kuja comes to destroy the earth and Goku/Djidane turns against his kin.
You kinda described Superman II as well...
And Guyver
And Kamen Rider
And Wall-E to an extent.
And Code Geass (just use the "foreigner" definition of "alien" in this case)
And The Iron Giant
And Stargate SG-1 (Teal'c)
And several arcs of Doctor Who

Let's just say it's not an uncommon trope.