>>Wonder if it creates a new marriage event only if there is **already** the same marriage event?<<
As we know frost, not every db reacts the same in the link/attach function, and I'm not about to test all of Ancestry's db collection. I did run a test using the same db I first discovered this severe wonky.
I started with a clean slate, using the same players, a female and two male spouses. I entered names and/or a non-specific birth year only. The bare minimum of data to generate hints.
As you can see from the examples, I linked/attached the marriage citation. It DID create the marriage event, and linked the two individuals, and created phantoms, AGAIN.
The dupes are only visible on the profile/overview pages, not in the main index, hence they're phantoms.