Xporter isn't assuming the templates pre-installed as already installed on the store.
Check the following steps to reproduce the error:
- Create a clean Jira instance and install the Xporter 5.8.2
- After this, perform the migration to the Xporter 6.0.0
- Go to the templates page and select the Store
- Try to install the template "Issue Details Excel", for example.
- You will notice you will have the same template installed twice.