If it was cast by Garruk's controller, he or she can choose which one stays and which one goes to the graveyard.) (Note that as Clone enters the battlefield as a copy of Garruk, it will have 3 loyalty counters, because that's a property of the Garruk card.) It was possible to have Clone remain on the battlefield even under the old rules, but it required a much more contrived set of circumstances
In summation, this card basically represents the one thing you can expect from Lorcana sets to expect the unexpected
The real answer here is slowly. Rather than dilute the canonical powers of such a godlike being, Wizards of the Coast has simply slapped on the kind of mana cost you would expect to pay for them
Normally, when an enchanted creatures leaves the battlefield, Auras attached to it will go to the graveyard as a state-based action
This means that there are two states of Common Cause it does nothing, or it gives all (nonartifact) creatures on the battlefield +2/+2