Galina tries to "hump" her brother all the time.. We joke to her- " Weenies you have it backwards child.."... lol.. ( no- I would NEVER breed her brother to her... it was a joke..)
If you tell the one that is the 'top dog" to stop.. Sometimes it does as you just put up the one that was "being humped".. However- my guess is that Duke is telling Rocky- look- you may be bigger but I am the boss... And I was here FIRST.. lol..