Look up "Multifeed" or "Separation" in the PDF index. You are directed to Chapter 7 (Feeder System) and Chapter 9 (Troubleshooting).
Your IR 2420 pulls multiple sheets of paper at once (multifeeding). Canon Ir 2420 Service Manual Pdf
The manual provides the torque specs (do not over-tighten) and the adjustment procedure for the paper height sensor. Look up "Multifeed" or "Separation" in the PDF index