For what it's worth, I tried a similar situation with Amazon's checkout form to see how they handle this situation. I found that on the final "place my order" form that you can submit in 2 browser sessions with different options (2 different shipping options in my experiment) and Amazon will take 1 cart and turn it into 2 orders with 2 shipping choices.
Conclusions and thoughts:
I guess this means webform follows a "first submit wins" strategy to the problem, whether on purpose or not. Knowing that alone is useful to me :)
I'm not sure what the *right* thing to do is or whether it's webform's job to protect against this. Without considering the technical details, I can imagine that it would be nice if webform saw a submission of a step then it would check to see if that step had already been submitted for that page and somehow ask the user whether they want to keep the old data or use their new data. I guess that idea would be best if there were a way to see the old submission vs. the new submission (like a diff) which is obviously also a hard problem to solve.
Closed: won't fix
6.2
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I think this is too complex to fix. If someone needs a webform submission locking feature, it should be done in a dedicated contributed module.
That seems OK to me. For what it's worth, the Webform Autosave → module was built to try to address this problem using optimistic locking. That feature has required careful tuning over the years.