I'm with Mr. Smith, you've got to figure out why this happened.
Questions: Are you using the rubber washers under the bottles and the brass washer under the washer on the powder side?
What size shot are you using? Shot could be hanging on a burr on the inside of the rammer tube at the bottom. If there is a burr, file and polish it out. Larger shot's worse. Do you have a small gauge rammer tube? My old 12 gauge Sizemaster tube is .560" at the bottom, and the new one likely are bigger.
Is there any chance little fingers twiddled the knob on the adjustable bar? Did you check powder weight during the cycle, while running a shell though? Just sliding the bar back & forth will throw light. Knock out a really tight primer can rattle the loader quite a bit, especially if it's a Winchester AA 20 gauge with some plastic in the primer pocket. I deprime them separately the first time. Add the snaps from the resizer and crimper releasing and you've got quite a bit of powder settling vibration.
According to my old chart, you should have your bar set between 7 and 8 for 20.5 grains of Green Dot. If you're throwing a double charge on a single stroke, it must be screwed out all the way to 15.
A baffle helps, but not having one doesn't explain a double charge.
Something doesn't compute.
Check out everything and get back to us.