Well, the RAW don't specify how the "repeating" function works. Art has varied from the historical Chinese lever-action bow, to one that has a crank, to one with a stack of five bows.
Realistically, the historic weapon has a large lever which moves the entire "magazine" section forward and back. A catch on the underside snags the string as it moves back, and another one pops the string out of the notch when it reaches the farthest-back position, stripping a bolt off the bottom of the magazine. So




-and-fire is one action, one pull of the lever. However, since you are holding the weapon with one hand and working the lever with the other, the bow must be "light" enough that you can




it one-handed. So a "heavy repeating crossbow", much less a repeating Great Crossbow, is not realistic. Also, since you are literally "shooting from the hip", accuracy is terrible. Historically it was a battlefield weapon - point it at a big mass of enemy troops and you'll probably hit something - not a sniper weapon.
But, if you aren't bothered by the existence of the Orcish Double Axe, you can handwave the existence of Repeating Great Crossbows too, I suppose.
(edit - ah, don't you love mindless, automated "swear word" replacement?)