mongos happily starts when you specify duplicate config servers for the --configdb parameter, for example:

mongos --configdb config1:27017,config1:27017,config3:27017 --logappend --fork

This is easily caused by a typo and can lead to incorrect and unpredictable behavior. In the case of an upgrade from 2.2 to 2.4, it will break the upgrade and manual intervention (resync of config servers, removal of lock, ...) is required.

Each address in the configdb string should be checked not just for string equality but should also be resolved and the resulting address should be checked as well. The set (with no duplicates) of addresses should be equal to the number of entries in the configdb string.