You can't take a feat multiple times, unless it states so in the "Special" section of the feat description, which Adopted Ancestry does not. So, this is just a bug in Pathbuilder.

I won't comment on this specifically, because I just don't know. But it certainly wouldn't be the first time Pathbuilder allows a technically-illegal combo. I noticed it allows you to pick a key ability that's available to any subclass of your class, not just your subclass.

I wonder if anyone has collated a list of all known cases of builds that Pathbuilder allows which the actual rules would not?