you wrote
In QtDesigner I add a signal/slot to a QPushButton where the Sender is the QPushButton (signal is clicked()) and the Receiver a custom slot (called on_pbAdd_clicked()) on my form class derived form QDialog.
that means, that "on_pbAdd_clicked" default slot according to this, in ui-file you can find such call QMetaObject::connectSlotsByName (this will create first connection), than you have declared slot on_pbAdd_clicked and probably connected it in ctor (I'm not sure) in that case you created the second connection.