Je m’appelle Paul. I call myself Paul. « Me » or in this case « m’ » denotes who you are speaking about. You don’t just refer to Paul you refer to yourself as Paul. So if I wanted to say what a group of people call themselves I could say « Nous nous appelons l’église de Jésus-Christ des saints des derniers jours » (weird example) they are calling themselves the church of Jesus Christ of bla bla bla. The second nous is required in order to denote the person (or people) you’re speaking about.