The postinst script for mongodb 2.4.9 has the following:

case "$1" in
    configure)
        # create a mongodb group and user
        if ! grep -q mongodb /etc/passwd; then
                adduser --system --no-create-home mongodb
                addgroup --system mongodb
                adduser mongodb mongodb
        fi

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.