Duplicate entries in civicrm_settings table cause unpredictable operation

civicrm_settings table entries must be unique but the groupname/name index is set to allow duplicates (it should be defined as unique).

Menu rebuild, which is ill behaved, will create multiple entries, only one having content and the rest having null as content, the menu display routine may chose the wrong one to display and display no menu.

Likewise when directory and resource URLs are updated it will occasionally create a second entry, which causes the old values to be picked up.