I did a test just as you described whereby I created both ribbon definition files from scratch and I could recreate the same issue (we are working with M020)
But notice that you can also manually create multiple tabs with the same name. So I assume that they use a kind of internal ID and that the name is only an attribute and not an unique identifier
But I found a way to get it right : try to create the ribbon definition files as follows
By following the above steps I didn't get any duplicate tabs no matter in which sequence I started both applications.
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