The postinst script for mongodb 2.4.9 has the following:
grep -q mongodb will succeed if any user exists that has "mongodb" as part of its name. In regex parlance, the grep is not anchored.
In particular, if the mongodb-mms-backup-agent package is installed first, it will create a mongodb-mms-agent user. Then if mongodb is installed after, it will fail to create a mongodb user as grep -q mongodb will match on mongodb-mms-agent.
You might consider running /usr/bin/id mongodb >/dev/null 2>&1 and checking the return code.