Double slashes are not the #1 single most common question posted here. But they come up often enough that you can search and find plenty of earlier threads. Do not try "//" as a search string; use "double slash".
This is assuming you are talking about legitimate human (or welcome robotic) visitors looking for real pages. The double or multiple slash inserted by unwated robots is a whole nother issue. But a Forums search will bring up both of them.
Make sure you deal with both parts of the problem: how to redirect appropriately, and how to trace the problem to its source.
Incidentally, a // at the very beginning of the URL-- right after the domain name-- may not even malfunction. Possibly it depends on your server. I've seen the occasional specimen in logs, complete with successful image requests. It's the // deeper into the URL that is really troublesome.
Rewrite (that's rewrite not redirect) any request with double or more slashes to a special PHP script that then detects the requested URL, uses preg_replace to fix it, and then sends a 301 header redirecting to the correct URL.
That's your short term fix.
The long term fix has to be to fix the bug in the site script that allowed the error to propagate.