Confirm Box - Server Side

Hello,
Confirm Box - Server Side and should be displayed based on condition

On Save Button, I have 2 conditions, one for updating the record and second for inserting the record. If the record is for inserting i am checking for unique record (combination of fields), if the record exists with the same details then i need to prompt the user with a confirm box yes/no, or Ok/Cancel, if the user clicks Ok then i need to insert and if he cancels then nothing has to be done, so now i cant add onclick attribute to button as I have conditions, how to capture the confirm result on server side. It would be very helpful for me to complete one of my page.