STEP 1: Think More Carroballista, Less Crossbow.
There is no way to keep such thing hand-portable and powerful enough for military usage because no matter what solution you use, you will need add a whole bow's weight to the design as a theoretical minimum as somewhere to contain the extra potential energy... but many historical crossbow like weapons were not hand portable anyway; so, this may not be an issue. The Ancient Greeks and Romans often used heavy crossbow like weapons that they could fire from chariots, wagons, ships, or city walls. Many of these were designed to be powerful siege weapons, but they stored many times as much potential energy as a standard crossbow meaning that you could modify this concept to make a multi-shot crossbow.
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STEP 2: Design the repeating mechanism
Once you eliminate weight as a major constraint, you can begin to consider designs that would otherwise be impractical. Crossbows with multiple bows would be one option, but too many bows bolted together and they will start to get in the way of aiming; however, one possibility would be to use a ratchetted drop weight mechanism. With this method, you'd have just one bow that redraws itself using the potential energy stored in a heavy drop weight. The way it would work is that it would fire the bolt, and when string hits the end of its stroke, it will hit a release on a ratchet that will allow a weight to drop. This dropping weight would then pull the resetting mechanism on the bow and the ratchet wheel would relock when the string falls into the setting mechanism. You could then use a magazine to set a new bolt allowing the weapon to fully reload itself. Unlike with multi-bow concepts, this weapon could fire many shots without making the bow itself overly cumbersome to work with. A 20 shot bow would be just as simple of a design as a 2 shot bow, it would just require more drop weights.