@ArnaudLMI know the overlapping date condition can be added to prevent submission. In the Audit Rule condition builder, there is I believe you would then just add a condition that designates the correct policy this would apply to.

Now, if you have more than one policy a user can select, you would need to make a second audit rule with the same overlapping date condition, but would then add a condition for the second policy.

I think this would work.

I always, and I cannot stress this enough, ALWAYS test new audit rules with test users first before activating them for your real users.

The rule would look like what is in the screenshot below. The policy in the example, is just a policy we have in our training site. Give this a try and let me know if it works.