Thank you for the quick reply John.
1. There is no web-standard prevention of having the same value for multiple options in a select field – it is a common use case.
2. In the “real” end, a post object field is just an array (not a select field), no prevention for the same value repating more than once here either.
3. From my understanding (pretty sure about this), each time you begin to add an item to the post object field the wp_query re-runs. Each re-run gives all of the options regardless of what was selected before. And it is a specific JS script that forcibly removes options that were selected before.
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