Can you use an Action more then once a turn?

This came up last night with Forest Dragon cause player played it second turn third turn if you can use the action twice he can do 12 damage turn 3

Yes, you can. If action doesn't trigger when some certain condition is met (Blood Dragon Knight: When this unit attacks) or isn't limited (for example Knights Panther: Limit twice per turn) , you can play it as much as you want. In your case as long as your opponent has enough resources he can double the forest dragon's power counting his actual (already doubled) power.

HOLY ****!!! with the the support card that gives him three resources for three thats does two damage to the capital a round turn three 12 attack buring a zone