Some French rules to help you:

The consonant m is never doubled, when it is followed by a consonant (atmosphere, temperature, etc.) or preceded by an é (émettre, émail, etc.)

Words beginning with am almost always take an m, except: ammoniaque (and its derivatives).
Words beginning with imm almost always take two m, except: image, imiter, imam (and derivatives).
Words beginning with om always take an m
Words beginning with comm take two m, except: coma, comédie, comestible, comète, comique, comité. (And derivatives).
Words beginning with dom take an m, except: dommage, dommageable.
Words containing ...emm.... always take two m, except when followed by the sound [é].
Words beginning with hom, or that contain the syllable hom take an m, except: homme, hommage, bonhomme.
Words beginning with somm take two m, except somatique, somatotrope, somation (homonymous with sommation).