[itsm] Adding attribute with unique content

Hello,   I need to add an attribute to all items. I know how to do this, but in my case the content entered within this attribute needs to be unique.   For example: If I enter a text like “testingpurpose” to this attribute, “testingpurpose” should not be allowed to be entered in any other item in this attribute (similar to the unique number given to each item upon creation).   Any help appreciated.   Kind regards, Tilman