check if the value of two textboxes are same, if same show error message

I want to check whether the values of the tow textboxes are same or not. If the values are same, display error message. We can use asp:CompareValidator Operator="Equal" to check if the values are same or not, but it gives message only when the values are different. But I want to display message when the values in the two textboxes are same.