What happens if something has first strike and is then given double strike?
It will have double strike. Double strike is first strike plus normal combat damage. Having two instances of first strike is redundant.
A creature with double strike deals damage twice once as first strike and then if its still alive deals damage a second time. So giving a creature who already has first strike double will benefit the creature but giving a creature who has double strike first strike is not changing the creatures damage.
RESOLVED