In previous versions of Automatic, the matching engine was a bit dumb when it came to duplicate items. If the option “Download every episode once” was enabled, it would discard all older duplicates and only match the latest (the reasoning being, the latest item was likely to be the best). However, matching a single item meant that if the link proved to be bad, there was no recourse (it kept trying the same bad link).
To improve this situation, the 2.1.5 engine will group up duplicate items, and will try to download them in succession. So if a link fails for whatever reason, it will try the next alternate, then the next, and so on, until it either downloads successfully or runs out of alternates. Note that this only applies when the “Download every episode once” option is enabled.