I create a plugin for 5.X and up.

But I think it must be done for the official version too. It happens when one user click multiple times on a link or sometimes when arobot check link before.
We have 2 solutions , both have issue

1. Add a locking mechanism : create a lock when checking token + survey
2. Unlock when a response is created (and add register_shutdown_function)
3. During lock process : send a 409/429 or wait some ms in a loop until it's OK.

There's another issue with this type of surveys: if two people open the same questionnaire with the same token, they overwrite each other's answers with every action, without ever reloading the responses.
But i think it must be managed by plyugin (core or not)

This is quite an effort for an edge case. It will also create alot of load on servers.
I don't really think it is worth it.

This is quite an effort for an edge case.

OK, then on a plugin, some companies really don't allow multiple lines for one data.

It will also create alot of load on servers.

Not if you send a 409 before all actions. Here are 5 actions at the same time. With a 409 : 1 action and 4 quick actions.

I could propose adding the plugin to the core afterward, with the option to choose between a 409 response and a delay.

Allowing only one person to load a specific token is great too for load on servers, and disabling person 1 sending data and person 2 sending other data (deleting person 1 update).