Warning message during Assignment update

Summary:

We are having a requirement wherein the HR/manager should be notified with a warning message when they are trying to update an assignment stating that there is already an assignment update made for the effective date selected.

We know this is not possible via application behavior as system can allow more than 1 update on any record for an effective date. We are looking for a possibility to show this message or notify via any custom method that to the HR in real-time if such updates occur. kindly let us know if you have faced any such related requirement in the past so that we can think of a custom approach to see if this requirement can be made feasible.