Hi @Alex_B_Centrifuge you don’t need to redirect duplicate urls. You can either just disable them from search engines (have a no-index on them) or just leave them as-is. Google just won’t display duplicates in search engines and if you put in a canonical that should resolve it fairly well.
You definitely need to redirect any urls that were in the old site and not in the new site so that should be quite straight forward - not had any issues redirecting folders.
You can also disable folders from search engines which will put a no index on it, then can have the data all display just on a page. For example, disable ‘/folder/’ and create a page ‘/folder’ and then it resolves the issue. Then all the data within a folder can still be viewed separately ie. ‘/folder/page’.
@Alex_B_Centrifuge yes I can see that is happening. I have tested putting in a redirect of the trailing slash to non-trailing slash and that does work. Interestingly, I can’t get a ‘folder’ to automatically list in the sitemap so it is hard to see how the system is actually dealing with these files on the sitemap. That being the case, if you use the url structure /folder for all your navigation then the /folder/ shouldn’t be an issue.