Once we upgraded to 4.6,  some posts are being submitted twice/duplicated. Even though the members submit once. It doesn't seem to be issue with all members, but definitely with some.

Double Post Example

Does this also happen when using the default theme and all plugins/3rd party apps disabled?

I am unable to access to take a look due to access details on file being incorrect, but as mentioned above please ensure you disable all 3rd party items and test this again. If you take a look in system logs also, as there may have been an issue at the time of posting that you can pin down there