Duplicate content issues are a well recognised bugbear in the world of SEO. Whilst you're probably familiar with avoiding duplicating normal content (e.g. page copy), perhaps you've failed to see that there is another technical issue that search engines will view as a duplicate?
Could you be falling foul of trailing and non-trailing slashes in your URLs?
Why trailing and non-trailing slashes cause duplicate content issues
Trailing slashes are URLs ending with '/'
If you add or remove a trailing slash on your active URLs and your website doesn't redirect to one version of the page (e.g. correct itself) then you could have a duplicate content issue.
These are all considered different pages by search engines if redirect rules / canonical tags are not set up correctly