Thank you for the report!

I was unable to reproduce the issue on my end, but a fix for a related race condition was included in v4.2.0.
Could you please test with v4.2.0 and let me know if the problem is resolved?

If you can still reproduce it, it would really help to know:
What you mean by "saved session" (Firefox built-in session restore, or a session manager extension such as Tab Session Manager?)
Which mode you are using: auto-close or notify only
Your Firefox version
The exact steps to reproduce

Also, if possible, could you open a GitHub issue? It makes it much easier to track and follow up: https://github.com/Peuj/duplicate-tabs-closer/issues

Thanks again!