The zodiac sign of the month Adar in a non-leap year is Pisces (Dagim). But what kind of Adar I, which is in a leap year? Two months cannot have the same zodiac sign.
According to this answer on Hidabroot, it's the first Adar that has the mazal of Dagim. The question should be what mazal the second Adar has. The answerer suggests that the reason that Dagim is plural rather than singular (dag) is because it can cover both months. However, there are views that the second Adar doesn't have a mazal at all (more sources can be seen here, section gimel).