This is not normal behavior. Are you sure that you dont have a script trigger running?
I've looked everywhere I can find for triggers but I'm pretty sure that none are active. I certainly haven't added any.
Well, what do you know ... I just re-checked the portal box and a few fields and the Browse was ticked in the Script/Triggers menu. Even though no scripts were configured - just enabling them within Browse mode seems to do something. I've unchecked the Browse enabler in the Script/Trigger menu and I can now, at least, browse without creating duplicate parent records!!
This now shows up another issue - which I'll re-post as a new thread.
Thanks for the heads-up on the script/trigger - even though none were configured to do anything!