opnURL with a NEW window works - but doesn't

opnURL with a NEW window works - but doesn't

I have a parent form where the user may select a Customer and (using OpenURL new window) can open a new window to view and edit that customer. I want the user to be able to go back to the parent form and select another user allowing him two Customer forms open.

However - when he selects the second customer from the parent and issues the openURL (New Window command) the previous New Window is refreshed with the new Customer - a second new window is not opened - why?