Htaccess: add/remove trailing slash from URL

Question

My website runs a script called -> WSS wallpaper script

My Problem -> I have been trying to force remove or add trailing slash to the end of my URL to prevent duplicated content and also to clean up my URLs.

I have tried all sorts and tried everything I could think of and loads from the interwebs but no such luck yet! It might be a quick fix but I have looked at it so much I am probably blind to something dead obvious.

So I present you with all my .htaccess code:

DirectoryIndex index.php

RewriteEngine on
RewriteRule ^download/([0-9]+)?/([0-9]+)x([0-9]+)/([^/\.]+) image.php?id=$1&width=$2&height=$3&cropratio=$4&download=1 [L]
RewriteRule ^file/([0-9]+)?/([0-9]+)x([0-9]+)/([^/\.]+) image.php?id=$1&width=$2&height=$3&cropratio=$4 [L]
RewriteRule ^preview/([0-9]+)?/([0-9]+)x([0-9]+)/([^/\.]+) wallpaper_preview.php?id=$1&width=$2&height=$3&name=$4 [L]
RewriteRule ^thumbnail/([0-9]+)?/([0-9]+)x([0-9]+)/([^/\.]+)/([^/\.]+)/([^/\.]+)/([^/]+) image.php?wallpaper_id=$1&width=$2&height=$3&cropratio=$4&align=$5&valign=$6&file=$7 [L]
RewriteRule ^cat/([0-9]+)?/([^/\.]+)/p([0-9]+) index.php?task=category&id=$1&name=$2&page=$3 [L]
RewriteRule ^cat/([0-9]+)?/([^/\.]+)/([0-9a-zA-Z?-]+)/p([0-9]+) index.php?task=category&id=$1&name=$2&sortby=$3&page=$4 [L]
RewriteRule ^cat/([0-9]+)?/([^/\.]+)/([0-9a-zA-Z?-]+)-([0-9]+) index.php?task=category&id=$1&sortby=$3&page=$4 [L]
RewriteRule ^cat/([0-9]+)?/([^/\.]+) index.php?task=category&id=$1&name=$2 [L]
RewriteRule ^tag/([^/\.]+)/([0-9a-zA-Z?-]+)/([0-9]+) index.php?task=tag&t=$1&sortby=$2&page=$3 [L]
RewriteRule ^tag/([^/\.]+) index.php?task=tag&t=$1 [L]
RewriteRule ^profile/([0-9]+)?/([^/\.]+) index.php?task=profile&id=$1&name=$2 [L]
RewriteRule ^profile/comments/([0-9]+)?/([^/\.]+) index.php?task=users_comments&id=$1&name=$2 [L]
RewriteRule ^page/([0-9]+) index.php?task=view_page&id=$1 [L]  
RewriteRule ^register index.php?task=register [L] 
RewriteRule ^lost-password index.php?task=lost_pass [L] 
RewriteRule ^links index.php?task=links [L]
RewriteRule ^news/item/([0-9]+)/([^/\.]+) index.php?task=news&id=$1 [L]
RewriteRule ^news/page([0-9]+) index.php?task=news&page=$1 [L]
RewriteRule ^members/([^/\.]+)-([^/\.]+)/page([0-9]+)? index.php?task=member_list&sort=$1&order=$2&page=$3 [L]
RewriteRule ^members index.php?task=member_list [L]
RewriteRule ^messages index.php?task=messages [L]
RewriteRule ^submit index.php?task=submit [L]
RewriteRule ^search/([^/\.]+) index.php?task=search&q=$1 [L]
RewriteRule ^search index.php?task=search [L]
RewriteRule ^submit index.php?task=submit [L]
RewriteRule ^r-([0-9]+)?-([0-9]+)? go.php?id=$1&ref=$2 [L]
RewriteRule ^r-([0-9]+)? go.php?id=$1 [L]
RewriteRule ^([^/\.]+)/([0-9]+)/([^/\.]+) index.php?task=view&id=$2&name=$3 [L]
RewriteRule ^news/([^/\.]+) index.php?task=news&name=$1 [L]
RewriteRule ^profile/([^/\.]+) index.php?task=profile&name=$1 [L]
RewriteRule ^news index.php?task=news [L]
RewriteRule ^page/([^/\.]+) index.php?task=view_page&name=$1 [L]  
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/\.]+)/([0-9a-zA-Z'?-]+)/([0-9]+) index.php?task=category&name=$1&sortby=$2&page=$3 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/\.]+)/([^/\.]+) index.php?task=view&name=$2 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/\.]+) index.php?task=category&name=$1 [L]


## www reslove ##
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]
## www reslove ##

## index reslove ##
Options +FollowSymLinks
RewriteCond %{THE_REQUEST} ^.*/index\.php
RewriteRule ^(.*)index.php$ http://www.epicwallpaper.net/$1 [R=301,L]
## index reslove ##

Answer 1

Right below the RewriteEngine On line, add:

RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)/$ /$1 [L,R] # <- for test, for prod use [L,R=301]

to enforce a no-trailing-slash policy.

To enforce a trailing-slash policy:

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*[^/])$ /$1/ [L,R] # <- for test, for prod use [L,R=301]

---

EDIT: commented the R=301 parts because, as explained in a comment:

Be careful with that R=301! Having it there makes many browsers cache the .htaccess-file indefinitely: It somehow becomes irreversible if you can't clear the browser-cache on all machines that opened it. When testing, better go with simple R or R=302

After you've completed your tests, you can use R=301.