For example, a dragon's breath weapon is either there or it isn't.
If the spawn has it at all, it's fully there; if it doesn't have the ability, then there's no BW to worry about.
In this case, since a Doppleganger's IA is to shape change at will, the spawn would also be able to do it (otherwise it isn't a Dopplegager).
It might suffer a degree of difficulty doing it (call it "You CAN do it, but you have to Concentrate to do so, unlike your parents.") but it's still at will.
Like it has to make a Concentrate Check, DC 10 in order to change, but otherwise it's at will as normal.
If it makes the DC, it makes the change; if it fails the DC, it fails to change shape with a frustrated grunt.
"Whatha...? I said CHANGE! AAAGHHHH!"
=-)p
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