Ask someone to duplicate the double overhand knot on the left and they will show you the knot on the right.
There is a difference
The trick knot (left) looks the same on both sides. Their attempt looks fine on the obverse side. However on the reverse side there is a flaw at point "a"
Tie the first overhand knot around the tip of your finger
Bring the Working End (right) around to lie in the groove of the first overhand and complete the second overhand by going under one strand.
Slowly slide the knot off your finger while pulling on the Standing End with left thumb and middle finger and the Working End with right hand. It helps to keep all the strands in place if you squeeze the center of the knot as you tighten