Does the Talmudic dictum
משנכנס אדר מרבין בשמחה (When Adar comes we increase in joy)
start even in אדר ראשון (Adar I) in a leap year?
Rashi in the linked Gemara says that the reason we are "marbim b'simcha" is because they are "ymei nisim k'mo Purim v'Pesach" based on which some conclude that Adar Rishon is not included, as no miracles happened in that time period.
The Levush (685:1) says that we do not increase simcha in Adar 1. The Sfas Emes says we do.
Some want to conclude based on the dating of a letter of the Chasam Sofer (shu"t C.M 20) that he held we do (the dating says "א׳ דר״ח אדר ראשון שמרבים בו שמחה").
R' Ephraim Greenblatt holds that it applies to some extent to Adar Rishon was well.
R' Yiztchak Zilberstein cites sources for both opinions but concludes somewhat tentatively that Adar I is also included.
See also here.
The Shevet Halevi 10:105:3 brings a whole discussion and seemingly concludes that there is an idea of simcha in the first adar.