[Zope] meaning error message

My postgres database tables are setup to have unique index therefore no duplicates are not allow. If duplicate occur the db will return an error. Problem is when i try to submit duplicate data to the db i get an ugly error message. Is there anyway i can output my own error message for example "this data exist on the db already, please do not submit again" ? I don't want that error message because user's won't understand what is talking about. thanks.