Duplicate page titles

When I am looking on webmaster tools from google, it is saying I have pages with duplicate title tags. I went to see which pages and it seems I have two URLs for each page now..

https://mywebsite.com/post
and https://mywebsite.com/post/?w3tc_preview=1

it is something to do with the plug in W3 total cache.

I am worried the dup content will be causing the crawling errors. Im not sure how i can stop this? (ideally with keeping the plugin as it speeds my site up so much)