How to avoid standard message in Sap Portal?

Hi gurus,

i'm involved in a problem and I need to go to you.

I have a standard Portal aplication to manage Travels.

In a case, I need to create 2 travel in the same date, but it's common it become me an error message like this:

"Trip data for the period ....... already exists". Message nr.56146

I need to avoid this message but I don't know if it could be possible or maybe should I repair standard code or enhance it?

I need yoy knowledge!!!!