Adding a warning message to a task form

Hello Appian Community

I'm upgrading a QuickApp to validate days off in my team: users fill a form with a start date, an end date and his current project team then when submitted a task is sent to supervisors to validate the day off request.
I'd like to create a warning message that would appear on the validation interface when at least 2 users have sent days off requests for the same date.
You can find an example in the mock up attached.

What I'd like to do is displaying a message when a validator reads a task matching the case above (as read-only forms). I don't know what expression to use.
I'm new to this so your help would be really appreciated :)

OriginalPostID-252909

WarningMessage_mockUp.pptx