[DOCS] Add FAQ about duplicate form submission

This patch adds a new FAQ entry. It explains
how to handle duplicate form submissions.

Resolves: #95074
Releases: main, 11.5
Change-Id: Ide89f57dc296ab629b9bb50d76d7896cb998c008
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73660


Tested-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: waldhacker <hello@waldhacker.dev>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: waldhacker <hello@waldhacker.dev>