The vast majority of subscribers have subscribed via this URL:
When this User Agent visits the site, it reports the number of subscribers and the feed ID as part of the UA string:
"Feedfetcher-Google; (+http://www.google.com/feedfetcher.html; nnn subscribers; feed-id=nnnnnnnnnnnnnnnnnnn)"
However the same UA also requests this other malformed URL:
from time to time, and it has a very small number of subscribers. This URL has amp; in it, not just the & part.
A few weeks ago a 301 redirect was installed to remove the "amp;" part of the requested URL. Google appears to follow the redirect requesting both URLs in turn; the one with "amp;" followed immediately by the one without it, both using the feed ID with the small number of subscribers.
The point is, the "faulty" URL continues to be requested, and now there are two separate feed IDs that request the exact same URL, one natively and the other redirected.
How long does it take for the redirected URL to no longer be requested? Does it require the subscribers to alter something in their feed reader, perhaps unsubscribe and resubscribe, or will things eventually update themselves automatically?