Can't think of any other scenarios where this would be relevant, but are you allowed to declare a target lock on a ship you're already target locked onto and just keep it there?
If Dutch already has a target lock, can he re target lock the same ship in order to trigger his ability?
No.
This should be stated somewhere in the rules/FAQ but you may not assign a TL to a ship you already have a TL on.
Another example of this could be that a ship with a Fire Control system and a TL on the ship it fires at will NOT gain a new TL on that ship unless it happens to spend its current TL first.
Core Rule Book, Page 9:
Target lock tokens are only removed if the locking ship either acquires a target lock on a different ship or spends the target lock during the Combat phase.
No.
This should be stated somewhere in the rules/FAQ but you may not assign a TL to a ship you already have a TL on.
Another example of this could be that a ship with a Fire Control system and a TL on the ship it fires at will NOT gain a new TL on that ship unless it happens to spend its current TL first.
True, but as stated in the FAQ, you can spend the TL and elect not to reroll any dice, thus triggering a new TL with FCS and then Dutch, to run with that example, could trigger his ability.
To answer the original question with full reference, check the FAQ, page 14:
"Q: If a ship already has a target lock on an enemy ship, can the locking ship acquire a target lock again on the same enemy ship in order to trigger a game effect (such as “Dutch” Vander)?
A: No."
Some subtle impacts that this has is on Captain Kagi (see entry of the same name in the FAQ, page 7)
(...)
"If a ship already has a target lock on Captain Kagi, it may acquire a target lock on a different ship."