Stuck in the supplemental index

Our site has URLs that can change every other month or so. As such when Google visits that URL it sees a very similar page to what it already has in its index and so it makes it a supplemental result. Problem is both the current and previous URL point to the same page and that may change a third, fourth, or fifth time.

Is there any way to specify a sort of "permalink" in each page so that Google will always know it is dealing with the same page in case it reindexes it under a different URL?

Currently our only solution is just to do a 301 permanent redirect so that Google updates its index.

Why does the URL keep changing? I'd find a way to stop that from happening if at all possible. In essence you would be creating a permalink as that page really should be accessed by only one URL permanently. Otherwise you're essentially starting from scratch every time you change the URL (and that is in addition to the duplicate content issue).

It changes partly because it can be reached by a short numerical "page_id" and also because the main link is generated out of an item's title. If the item has its title changed to enough of an extent then the link is also altered to match the new title.