There is no problem with your Redirect directive as you posted it.
Look through your raw server log for instances of requests which may trigger this problem. It's likely that these double slashes are part of the original incoming requests. And such requests are often sent by clients with malicious intent (vulnerability scanners, scrapers, etc.)
If this is not the case, then these problems are likely being caused by mis-typed links (possibly on Web sites other than your own). Use the RewriteRule or RedirectMatch directive instead of Redirect, and simply allow for one or two slashes preceding the requested URL-path, discarding the extras.
In order to prevent many potential difficult-to-correct problems, do not mix the use of RewriteRule with either Redirect or RedirectMatch. Doing so means that you cannot control the execution order of these directives. Code is executed in per-module order, not strictly in the order that you place it in your config or .htaccess files.
Each Apache modules "scans through" your config files, and executes only the directives that it recognizes. The server configuration controls the module execution order, and thus the order of execution of the directives in your config files. Applying your directives out of the intended order can have disastrous effect on the operation and search ranking of your site. For this reason, if your use RewriteRule for anything, use it for everything.
You also want to be sure that across all config and .htaccess files, all external URL-to_URL redirects are invoked before any internal URL-to-filepath rewriting can be invoked.
The .htaccess file is processed line by line by mod_access which will use only the directives it understands. The file is then processed by mod_alias and again it will use only the stuff it understands. After that, mod_rewrite will process the file line by line using only what it understands. I mention just those three modules here, but in real life there are at least half a dozen more that will also have a go, and the ordering of the modules might not be exactly as I have described.
As well as being sure that across all config and .htaccess files, all external URL-to-URL redirects are invoked before any internal URL-to-filepath rewriting can be invoked you also need to be sure that for any request that is redirected, there is a maximum of one redirect from original request to final URL. That is, no request should ever result in an unwanted multiple step redirection chain.