if an employee is already assigned as a replacement during a specific period, they should not allow

We have enabled a Descriptive Flexfield (DFF) named Replacement Employee on the Add Absence page, and it is set as mandatory while applying for leave.

Our customer has a requirement based on the following scenario:

• Employee 1 (Emp1) applies for leave from 2nd June 2025 to 5th June 2025, and selects Employee 2 (Emp2) as the Replacement Employee in the DFF.
• Now, if Employee 2 (Emp2) attempts to apply for leave during the same period (2nd June 2025 to 5th June 2025), the system should prevent Emp2 from submitting the absence request.

In short, if an employee is already assigned as a replacement during a specific period