Fix silent ignoring of superfluous save retries

[fm3]

A mistake in #3829 broke the feature introduced in #3767 so users with spotty internet connections could provoke wrong 409 CONFLICT errors.
This PR fixes the lookup of the handled group ids

Steps to test:

• normal saving should work
• we adapted the front-end code to send “retries” even if the first request succeded, and tested that it can now recover from that agin
• Another way of testing this was described in Silently ignore duplicate tracing save requests #3767 (comment)

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• Updated (unreleased) changelog
• Needs tracingstore update after deployment
• Ready for review

[youri-k]

Nice catch 🎉