How Double Feed Detect Works
Double feeds are detected by sensors mounted above and below where the paper feeds through the scanner.
These sensors send a signal to each other. The signal passes through one sheet of paper just fine. If the the sensor passes through the first sheet, and then a second sheet, the scanner will stop and let you know you have a double feed detected.
There IS paper in the scanner, you just can't see it. The sensors are detecting paper. It's possible that you have a failed sensor (<1%), but it's much more likely you have a blocked sensor.
Look at the small sensor "holes" in the diagram to the left. Very easy for a tiny shred of paper to fill one of those holes.
You need a can of compressed air to "blow out" the paper and dust in each and every sensor hole.
If that does not work, it is possible that the paper or dust spec is firmly wedged inside the hole and the only way it will be "cleaned" out is for a certified scanner technician to come in and remove the sensors (from the inside out) and clean them. This is NOT an easy process but one that is covered under a regular manufacturer service contract.
On Site Service
If your sensors are blocked, and your sure there isn't any paper that you can see in the scanner, then you will likely need the services of a scanner technician. If you are not under a service contract, a "One Time" deep cleaning and service might set you back $500-$800. An annual deep cleaning will also help maintain the life of your scanner, get rid of squeaks and creaks and improve the overall performance of the scanner.
Blocked sensors are NOT covered under a warranty, broken sensors are, but they RARELY fail. The culprit is almost always paper chad or dust in the sensor hole. Only a service contract visit will take care of it. Most high speed scanners have a service contract that includes 1, 2 or 3 deep cleanings (on site) per year.
Proper maintenance of the scanner will resolve most issues and prevent other issues from arising. Please refer to the maintenance section of the scanner manual provided with the scanner for more detailed cleaning instructions.
for scanner cleaning kits and parts.