Remove trailing slash wordpress website URLs - wordpress

Question

I have wordpress permalinks set up to /%postname% (note there is no trailing slash at the end).
However, wordpress is not redirecting any given page URL from the version with the trailing slash to the non trailing slash version. Any website header checking tool would return a '200' (OK) code for both versions (with and without the trailing slash). I am concerned this will have negative SEO consequences due to duplicate content. Therefore I want to have only one version of each page.
I am trying to modify the .htaccess file located on the wordpress directory so it removes the trailing slash at the end of any URL pretty much redirecting to the non trailing slash version of the URL.
I have Wordpress installed on a sub directory not on the root of my site i.e.
http://mywebsite.com/wordpress/
I tried the following code which I compiled by reading different questions posted on this website:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /wordpress/
RewriteRule ^index\.php$ - [L]
RewriteRule ^(.*)/$ /$1 [L,R=301]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /wordpress/index.php [L]
</IfModule>
The problem then is that I cannot login to wordpress admin. When I enter my user name and password on mywebsite.com/wordpress/wp-login.php
it typically used to redirect and enter to mywebsite.com/wordpress/wp-admin/
But now since the trailing slash is being stripped away from /wp-admin/ by the code I added to .htaccess then I get an error and cannot login to Wordpress.
I kept reading over this forums and then tried the following by playing around with the code and now I can login to wordpress but now the trailing slash is not being removed from any given URL.
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /wordpress/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_URI} !^/wp-admin/
RewriteCond %{REQUEST_URI} ^/wp-login\.php(.*)$
RewriteCond %{REQUEST_URI} ^/wp-admin$
RewriteRule ^(.*)/$ /$1 [L,R=301]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /wordpress/index.php [L]
</IfModule>
Can anyone recommend what else I could do?
Pretty much all I want if for page URLs not to have a trailing slash at the end of the URL with the exception of the wordpress login page (mywebsite.com/wordpress/wp-login.php) and mywebsite.com/wordpress/wp-admin/
Keeping in mind the Wordpress install is on mywebsite.com/wordpress/ and not at root level.
Thanks so much!

Answer 1

Try these rules:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /wordpress/
RewriteRule ^index\.php$ - [L]
RewriteRule ^((?!wp-admin/).+?)/$ /$1 [NC,NE,L,R=301]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . index.php [L]
</IfModule>