Dual citizenship: which passport counts for the pension refund?

The citizenship that keeps German pension routes open is the one that counts. If one of your passports is German, you are treated as a German citizen: voluntary contributions stay open to you, and a refund is blocked, second passport or not. The same logic applies if you hold the citizenship of an EU or EEA state, the UK or Switzerland next to a non-EU one: you are treated like an EU citizen, and a refund generally only becomes possible at retirement age if you never reached five contribution years. Only if all of your citizenships are non-EU does the standard route apply, treaty rules included. Your current residence is checked on top of that, so a dual-citizen case is worth running through the free Pension Check rather than guessing.