You can add some additional code to your
findselectdialog statement to do the job. Definitely a hack but it seems to work. If the control key is held down when you double click the item in the find/select dialog or press any of the buttons, this will go back to the dialog library file and get the value returned by the liveSelectChoice variable which will still have the contents of the record that was selected in the dialog. At the very end of that value is the server record id which can then be used to select only that record.
if info("modifiers") contains "control"
If you click the Select button in the dialog it will go back to your data sheet and simple select the record you had selected in the dialog. If you double click the record in the dialog and it is not currently selected in the data sheet it will ask if you want to add it. Regardless, it will simply select only that record. This will treat everything as a Select including the Find and the Cancel buttons if the control key is held down. As I said it is a hack.