.htaccess – Redirect duplicated post ended in ‘-number/’ to the same url without the -number/

You’re close. Add the following to your .htaccess file in between the <IfModule mod_rewrite.c> tags that were created by WordPress:

RewriteCond %{HTTP_HOST}
RewriteRule ^(.+)-[0-9]+/$ /$1 [R=301]

Your .htaccess should looks like the following if it hasn’t been modified by another plugin:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# Custom Rewrite
RewriteCond %{HTTP_HOST}
RewriteRule ^(.+)-[0-9]+/$ /$1 [R=301]
</IfModule>
# END WordPress

As a result it will do the following:

http://example.com/category/post-title[-NUMBER]

Redirects to:

http://example.com/category/post-title