t to obtain a security clearance for a government job , if my dad uncle has a double citizenship
Won’t be much of an issue as long as he isn’t a spy. Just be clear to document it. The investigator will want a good reference to verify so be sure to provide information on someone that can verify who that person is, their relation, where they live and their occupation. The reference needs to be someone that you are NOT related to.
It depends the nature of contact between you and that person. Either it is your uncle or your dad uncle AND you have frequent contact with this individual, who is a dual-citizenship person, you have to listed the person on the SF86, Section 19 - Foreign Contact. I would not list that individual if you have never talked to him or barely see him at all.
Had a friend whose uncle lives overseas, is in a foreign military and it was the main aspect of his subject interview (and the subsequent interview they had with me in trying to confirm the information). He never talks to him, and had only met him maybe twice in his life. They will find this information. Best to provide it and detail everything you know.
For the case of knowing someone serving or served in foreign military, I can see it questionable by the BI on that subject. I’m not sure the OP meant dad’s uncle or his dad-side uncle … lol … but I’m objecting your point about knowing someone or having someone in the family serving or served in the military, especially if they are a ranking member. . I think they should have the access to that record because the uncle had to declare his background as part of U.S. naturalization process.
It really depends on the Agency your are applying and which country is the second citizenship.