When confirming a segment or trying to run a verification, you receive the error:

An item with the same key has already been added.

 

1. Close Trados Studio
2. In Windows Search, type SDL Plugin Management and open it
3. Check if you do not have duplicate plugins installed.
4. Uninstall one of the plugin instances
5. Open Trados Studio and check if the error persists

or

1. Go to Add-Ins tab > Plug-Ins tab and see if any of the installed plugins appear twice
2. Un-tick one of the duplicated plugins, check if the error persists
3. Close Trados Studio and uninstall the plugin from RWS Plugin Management
5. Open Trados Studio and check if the error persists

Notes: This issue occurs as well on Tridion Sites - Translation Manager
 

Same plugin installed twice.