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?
Yes. I tried redirecting for several months. The requests for the wrong URL version continued several times per day.
For the last month or two I have been rewriting the request to a static XML file with one message in it "Please unsubscribe from this feed and resubscribe using the URL version without amp; in it". The requests continue and the subscriber number remains static.
I also tried something else for about one month in between those two, but can't remember what it was.
Feedfetcher does not follow robots.txt guidelines.
If your feed is publicly available, Google can't restrict users from accessing it. One solution is to configure your site to serve a 404, 410, or other error status message to user-agent Feedfetcher-Google.
It's understandable that you don't want to lose the subscribers. Maybe adding a large eye-catching image to the Please unsubscribe message might draw more attention.