To form most adverbs, we generally take the feminine form of the base adjective and add "-ment".
heureuse - heureusement, claire - clairement etc.
However, if the masculin form of the base adjective ends in "-ent" or "-ant", we have to double up the "m" for the adverb ending "-mment".
récent - récemment, suffisant - suffisamment etc.
The double "m" doesn't change the pronunciation at all, it's just one of those lovely spelling exceptions you have to deal with!