After reviewing the notes in #1722084: Need clarification on how uniqueness of a machine name is considered in this module, I've verified that adding more than one instance allows you to create duplicate entries. I can't immediately think of a use case in which multiple "unique identifiers" would be useful, but I'm not ruling it out. The field needs to check against itself in addition to checking against already-existing db values.
I'm going to start working on a patch.
Comments
Comment #1
c-logemannMaybe we need an own validator for this but just an idea. What about using unique_field?
http://drupal.org/project/unique_field
Comment #2
danrodComment #3
danrodI'm closing this issue given that Drupal 7 is no longer supported and for those who still use the D7 version of this project, the unique_field module should be a good option instead.
Comment #4
danrod