warning when the trip for the date already exist

hi friends

In the travel. When the trip is applied for a particular date when i again try to apply the trip for the same date it shows only waring but actually it should be a error. I want to make the warning into error. How to make that which BADI is called. Since am applying the trip from my portal. I want to display the warning into error to portal. Which BADI or Function Module is evolved .

Regards

vijay