Add Page Before Process Script

The little script below lives in my Orders - Add Page Before Process Event. It's just a counter which runs perfectly when the Add button is clicked, the projnum increments in the ProjNums table. But then, the script appears to run again when I click the Save button to save the new Orders record, incrementing the projnum a second time. The Orders and ProjNums tables are not connected in any way. ProjNums has only two columns, groupid and projnum with no auto-increment fields. The same double increment happens if I set up two similar tables in a sandbox outside my main project.
What's going on? Is this a bug in PHPR 8 or am I missing something?
global $dal;

$tblProjNums = $dal->Table("ProjNums");

$groupid=$_SESSION["groupid"];

$sql = "UPDATE ProjNums SET projnum = projnum+1 WHERE groupid = '$groupid'";

$rs = CustomQuery($sql);