And what is the relationship that you have set up between the layout's table and the portal's table?
Do you have any scripts that you run.
Both scripts, keyboard short cut, the menu option for new record can all create a new record. In some circumstances entering data into the blank bottom "add" row of a portal can create a new record--just to name a few possible methods. Entering data in a blank field from a related table might also create a new record in the related table in some cases.