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Ideally, you should use both, but I realize that could be a lot of work. Canonical URLs will work well for the case you mentioned above. Just remember to link to your canonical URL on internal pages and have inbound links point to that canonical URL. You should ask site owners that host those inbound links to change if possible or use 301 to redirect those links that can't be changed. You may also want to consider creating redirect rules to add or remove the trailing slash for all URLs, because links with and without the slash are considered different URLs and will split link juice.