This is the expected pronunciation, along with [co.mɑ̃.dɑ̃] and variations in the way /ɑ̃/ is realized.

Most nasal vowels directly followed by a nasal consonant (i.e. N or M) lost their nasality several centuries ago.

There are rare regional exceptions (See How is /a/ pronounced before n/m in French?) and a few other ones, as Gilles rightly pointed out in his answer.

This is French. Rules have exceptions. A vowel followed by N or M which is followed a different consonant or at the end of the word becomes a nasal vowel. There are a few exceptions: -mn does not lead to nasalization, the final -um (or -ums) is pronounced /ɔm/, recent imports from foreign languages are not nasalized, and probably a few more I can’t think of right now.

A double consonnant does not nasalize the preceding vowel except in the few cases where it does. The only exception I can think of is when the prefix -en was added to a word beginning with n or m (in the latter case, the word is spelled emm-), with some fluctuation when en- was derived from the Latin prefix in-. Here are a few examples which are nasalized:

ennui /ɑ̃.nɥi/
enneigé /ɑ̃.nɛ.ʒe/
emmener /ɑ̃.mə.ne/
remmener /ʁɑ̃.mə.ne/

But in other words the usual rule applies (e + double consonant is pronounced with an open /ɛ/ (è sound), occasionally a closed /e/ (é sound)):

ennemi /ɛ.nə.mi/
renne /ʁɛn/
flemme /flɛm/
emmental /ɛ.mɛ̃.tal/ or /e.mɛ̃.tal/
Emmanuelle /ɛ.ma.nɥɛl/ or /e.ma.nɥɛl/

Also emm is pronounced /am/ in femme and in adverbs that end in -emment.