There is no limit on the number of times you can use Margaery's ability in a challenges phase (if there were, it would have to be explicitly stated, as it is on many other cards). Provided you can pay the cost, "Kneel Margaery Tyrell", you can gain the effect. So it's perfectly legal to kneel Margaery for her ability, stand her (with Magister Illyrio, Handmaiden, Seal of the Hand, etc.), and then use her ability again.
Can you use the ability on herself ? — zothommogh 1