Where have you seen such a global?

Keys in global are unique. If you set the value of a global for the same key you'll override the value for the key.

Like @Eduard Lebedyuk wrote if you run 3 commands:

Set ^Data("Athens") = "5"
 Set ^Data("Athens") = "6"
 Set ^Data("Athens") = "7"

You'll first have 5, then override it with 6 and end up with value 7 for the key "Athens".