I didn't mean "dont use array questions", i mean "restart browser every time". Arrays don't cause problems, they can notify that something went wrong instead.

I've seen this happen too - we have had a few participants who completed the survey twice (open survey). Survey is:

All in one
Set cookie to prevent repeated participation: is off
Participant may save and resume later: is on
No tokens (open survey), but:
Enable token-based response persistence: Off
Allow multiple responses or update responses with one token: Off

LS version 2.73.0, build #171219

One participant provided a web history report to show that they completed the survey twice about a week apart. And we have no record of their response in our response table.

The survey is a short one with only 10 questions.

Link we've shared for the survey doesn't use newtest -- just the link that appears in the Survey Summary page...

We are using database for sessions, and have the session lifetime set to 14440 (~4 hrs)

What is the survey mode? All in one? Group by Group? Question by question?

Because for Group by Group and Question by question as soon as they pass the first page something should be saved and the session can't really be the problem.

But: if this is a bug it most probably will not be solved for the 2.x version, as it is discontinued.