Behavior on mentioning a "mirror name" twice in the thirdpartymirrors file (section 3.4.2) is undefined (although it hints that the first one is used). As far as I can tell portage 2.1.2.3 actually treats these as incremental (but iirc older portages do not and pick either the first or the last one, I forgot which). It is probably best to require each "mirror name" to occur at most once in the thirdpartymirrors file.
r155