How to Show my Own Error Message Instead of Mysql Error?

Dear all i made a Registration Form for user to input their Data, but i also had some Field Check before the data can be insert to the Mysql_Database! I had a question here, sometime the mysql shows the error : "Duplicate Key for xxx" I know what is this about, reguarding to my Registration Form, it mean the Login Name is Duplicated! But i want to show my own message to the user for this error instead the Mysql Error! It is meanness to show User the Mysql Error, cause they won't understand it!!! Could Someone pls tell me how i can do this? -- Thx a lot! Jack nedcor4@nedcor.com.hk