What happens if Manaroo transfer a blue target lock token to a ship that already has one?

That ship loses his original TL, unless he has an ability that allows him to maintain more target locks.

Rules Reference pg3 under Acquire a Target Lock:

"A ship can maintain one target lock. If a ship acquires a new target lock, it must remove its old target lock."

Special exception for Weapons Engineer, but he still ends up discarding one because he automatically acquires an extra.

Also directly answered on page 22 of the latest FAQ.

"Q: When an ability assigns a blue target lock token to another ship that already has a blue target lock token, does the receiving ship have to remove the previous target lock?
A: Yes, unless the receiving ship has the ability to maintain more than one."