Some time ago I filed gnatsweb PR 507 describing at least one mechanism
by which this can happen. Maybe this is what you're seeing?
1. File a PR the normal way and submit it. A confirmation page is shown.
2. On this confirmation page, click the "Create" link at the top of
the page. On the new Create page, look at the page source... note
the return_url defines the PR filed in step 1.
3. Fill out the second Create form and submit it. A confirmation
page is shown, which after the refresh period submits the
"return_url", effectively refiling the first PR and displaying a new
confirmation page.