Recently, while playing the new caprica expansion, we had a situation where there was a human player already in detention and a crisis card was pulled that resulted with the effect of "move a human character on new caprica to detention" My question is can the character who was already on there be selected to move there? thus not choosing anothee character on the planet? 

the verbage said any and not " not in there already".

Nope... though this rule is technically for the Brig it applies to Detention too...

From the FAQ, page 2:

Q: If the a player is required to choose a character to send to the “Brig” or “Sickbay,” can he choose a character that is already there?
A: No. He also cannot choose a revealed Cylon player, or “Helo” before he is on the board.

Ya know, I read these forums and have often wondered why some people ask questions where the FAQ has an obvious answer.... Well it seems i am in that boat now! I know i read that section but must have missed it! lol.

Thank you for shrinking my ego! :D

Lol, we all miss stuff from time to time... I think I've played too much of this game cause I usually know the answers to such questions even if I don't know where to find the answer between the FAQ and three rulebooks.
And you should see some of the stupid questions I ask about AGoT...