Forum:How long does Double Summon last?

My friend said that it lasted until it was destroyed, and that it stayed on the field until it was destroyed/sent back to hand/etc. I really doubt that this is true, otherwise this card would be a little broken, but he claims that it stays on the field due to the fact that it says that it can only gain the effect once per turn. Personally I think this just means that it cant be reused in the same turn, but who is correct?