It is not possible to prevent duplicate UI Action insert submissions without preventing legitimate subsequent task creations.
There was a previous partial fix in Kingston and late Jakarta, where the UI Actions handling code had been amended to trace the las action performed and attempt to inhibit repeated submissions.
Steps to Reproduce
This issue is not constantly reproducible, and depends on the network latency affecting the user client session.
When the user clicks on 'Submit' or 'Create New', the request would POST and the UI Action would get disabled.
While the new record is being inserted, after a period of network latency affecting the client, the button would get re-enabled.
If the user clicks on it again thinking the previous insert has not initiated, this would trigger the exact same POST as before, trying to create the new record with same number and sys_ID, violating the unique key indexes.
This problem is under investigation and targeted to be fixed in a future release. You can Subscribe to this article to receive notifications when more information will become available.
Related Problem: PRB1265879