Well snakes do not "turn in to" anything
If I am reading you correct you are asking what the progeny of a het x het breeding would be (assuming the animals are both het for the same trait.)
So to make it easy I will just use albino.
het albino x het albino will give rise to:
albino (1:4 odds)
het albino (1:2 odds)
normal (1:4 odds)
The hets and the normals will look the same though, so any given normal looking baby from the litter grabbed at random will have a 1:3 chance of being a het.
And yes, line breeding is common