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 plugin: Extension name" error.

To resolve the issue, go to Dewesoft Addons/Addons64 folder and remove all the files which are not in the subfolders.

..\Bin64\X3\Addons (or Addons64)

This is how the addons folder structure should look like: