family: deselect node if selected twice

Previously if a node was selected twice, it would compute the result
with the same nodes, which is now changed to be deselected if the same
node is selected twice

Instruction: Change instruction to "Select one of the pairs" since more
than one pairs are possible

Signed-off-by: Rudra Nil Basu <rudra.nil.basu.1996@gmail.com>