If you place a sticky piston and then extend it, then place a pistons or sticky pistons on the face of the other sticky pistons and try to retract the sticky piston, both pistons retract, now extend the first sticky piston again but this time extend the second piston also, it should look like a double piston extender. Now try to retract only the first sticky piston. I will attach a screen shot of the two pistons.