Error when adding multiple same class xml objects

[ray8828]

Thanks for sharing this great job!

When I try to add multiple same class XML objects e.g. two BottelObject to an environment, I got an mjcf XML error, (I've already specified different names for the objects)

Failed to load XML from string. mj_loadXML error: b'Error: repeated name in body array, position 25\nObject name = bottle_main, id = 25, line = 267, column = 7'

[cremebrule]

Thanks for catching this bug! This is fixed in the most recent PR.