If it should happen again, as soon as you have that first leg put a cord round it.
Then feel up the leg, never losing contact up the shoulder/hip round the back of the head/bum and down the other side. Unless you have Siamese twins then you can be sure the legs belong to the same lamb
Also front legs, the joints all bend the same way, hind legs the joints go one one way and one the other
Well done, go have a nice cuppa