how to add a confirmation box prior executing an script

I have the following script before adding a new record , to search for dublicates .

I need to provide a confirmation box to allow dublicates if user is press 'yes' or to return false

the script if the user press 'no'. please help.
global $conn;

$strSQLExists = "select * from _tblcertificatespro where Vessel='".$values["Vessel"]."' and Certificate='".$values["Certificate"]."'";

$rsExists = db_query($strSQLExists,$conn);

$data=db_fetch_array($rsExists);

if($data)

{
return false;

}

else

{
return true;

}