Hi,
I was wondering why the double 'mm' was used in older spelling such as in 'cumman'.

M is tensed anyway, so there would seem no need for m and mm seperately (unlike rr, ll, nn)

Or was there once a difference felt?

Do you mean, in Old or Middle Irish? I don't think was used in Classical Irish or later.

In Old and Middle Irish, was used because most of the time, "m" standing alone was pronounced as a nasal v (written mh inniu).

So we have Old/Middle Irish mm > Modern m
Old/middle Irish m > Modern mh