With the Dewesoft X3 SP5 installer or above we have made some changes in the extensions. Every extension in the addons folder is now saved to a folder. 

If the old version of Dewesoft was not completely removed or the extensions are still in the addons folder, Dewesoft can show "Duplicate extensions found:" error.

To resolve the issue, go to C:\Program Files\DewesoftX\Bin64\Addons64 folder and delete one of the folders with duplicated extentions.

This is how the addons folder structure should look like: