havent noticed this wording before so:

Action: Focus this unit to have a target unit gain [unit Damage] until the end of the phase.

does this mean you can only use this action if this card has no focus tokens already.

Rogue 3 said:

That's exactly what that means, yes.

To back up that answer, check pg 27, under "Exhausted:"

A card that is exhausted (i.e. has one or more focus tokens on it) cannot be focused to pay for card effects , cannot focus to strike, and cannot generate resources. However, other card abilities of an exhausted card may still be triggered, and its constant effects (if any) are still active, as long as such abilities do not require the card to focus, to be ready, or are not otherwise prohibited by the card text.