remove //xx after directory in URL

To remove //<something> at the end of the URL-path (eg. /podcasts//rebt to /podcasts, try the following instead at the top of the root .htaccess file:

RewriteEngine On

RewriteCond %{THE_REQUEST} \s([^?]+?)//
RewriteRule . %1 [R=301,L]

THE_REQUEST server variable contains the first line of the initial request headers (eg. GET /podcasts/rebt HTTP/1.1) and does not change when the request is internally rewritten (unlike REQUEST_URI).

The regex \s([^?]+?)// captures the part of the URL-path before the first instance of a double slash. Anything after and including the double slash, are discarded. This regex also ensures we do not inadvertently match against the query string (if any).

The %1 backreference contains the captured subpattern (ie. everything before the first double slash in the URL-path) from the preceding CondPattern.

Aside: Note that this will not work properly if the preceding URL-path maps to a physical directory, since it will result in two redirects. eg. /directory//something to /directory to /directory/ (by mod_dir). In this case, you should avoid removing the first trailing slash.

You should test first with a 302 (temporary) redirect to avoid any potential caching issues and only change to a 301 (permanent) redirect when you are sure it's working as intended. You should clear your browser cache before testing.

A look at your existing rule...

RewriteCond %{REQUEST_METHOD}  !=POST
RewriteCond %{REQUEST_URI} ^(.*?)(/{2,})(.*)$
RewriteRule . %1/%3 [R=301,L]

This code is intended to reduce multiple slashes to single slashes in the URL-path, not remove the double slash and remaining path entirely. eg. /podcasts//rebt to /podcasts/rebt. However, since it checks against the REQUEST_URI server variable (which can change throughout the request) it may not work as intended.

Also, the condition that checks against the REQUEST_METHOD would seem to be redundant, unless you are erroneously POSTing to double-slashed URLs internally? A 301 redirect removes any POST data (since the browser converts it to GET) - hence why the check may be necessary in certain cases.