Use Canonical Tags to Resolve Trailing Slashes
Canonical tags are the best way to deal with trailng slash duplicate pages.
Don’t Use Noindex on Canonicalised Pages
Don’t mix Noindex with canonical to a different page.
Many to One Canonical Tags May Be Ignored
Google will try to follow canonicals by default, but they ignore canonical tags if there are significant content differences, or if a lot of URLs canonicalising to the same page, if they think it’s a mistake.
Publishing multiple versions of a page on different sites, and canonicalising to a single page for link building, is not acceptable. The duplicates should be nofollowed.
Google May Choose an HTTP URL as Canonical if the HTTPS has Mixed Content
Google will try and choose a canonical URL which does not have mixed content issues from http/https. I.e. if there is an https version of a page that DOES have mixed content issues, but an http version that does NOT have mixed content issues, then the http version could be used. If however, there are very strong signs to use that https version coming from the website, (such as redirects or rel=canonicals) then Google will still use the https version even though it has mixed content issues.
Don’t Mix Noindex and Canonical
Having a page with a canonical tag pointing to a page with a noindex is a problem but the canonical might just be ignored.
Canonicalising Product Variants Loses Unique Content
If you have product variant pages, instead of noindexing, you are better to canonicalise the variations to a single version which can consolidate all the ranking signals. However you will lose any unique content from those variations.
Googlebot Can Be Redirected to Canonical URLs
John says that althought it’s technically cloaking, it’s actually OK to redirect Googlebot from URLs with tracking parameters to canonical URLs, but allow users not to be redirected, so they can be tracked in analytics.
Duplicate Pages with a Noindex May Be Selected
If you have duplicate pages with the same content which aren’t canonicalised, but one of the pages has a noindex, then Google might pick the noindex version, and then that page will be noindexed even if there is an indexable duplicate.