Instead of using canonical tags between these two URLs, trying implementing 301 redirects.

If you can 301 redirect `/send-gifts/E6/` to `/send-gifts/e6/`, Google would certainly stop reporting duplicate titles between the pages after a full recrawl.

I would have expected canonical tags to do the job, but 301 redirects certainly will.

If all of your canonical URLs are completely lowercase, then this question should help you implement the redirects: https://webmasters.stackexchange.com/questions/18670/how-to-redirect-any-url-to-lowercase-url