A common cause of 'duplicate content' is where you have multiple versions of the same URL.
If Google has indexed all of
separately, that could give you a problem, because it is treating them as four separate pages, rather than as four routes to the same page.
What should happen is that whichever one you type into the address bar, it gets corrected to your preferred format. It doesn't matter which of them it is (although it's good practice not to include 'index.htm'), as long as it's consistently applied throughout the site and each actual page only shows up with a single URL.