This isn't a new bug, but figured I'd report it since I haven't seen it corrected in a few versions. In domain-mapping.php on line 37 it says:
// UnComment out the line below to allow multiple domain mappings per blog define('DOMAINMAPPING_ALLOWMULTI', 'yes');
This implies that this should be commented out by default, but it isn't. It makes sense that this is commented out by default because a minority of networks use multidomains.