Fujitsu fi-4220C Operator's Manual page 91

Check length
Check thickness and length
Furthermore, see "8.5 Double-feed Detection Conditions" for
detailed information about the document for double-feed
detection.
4.
Click the [OK] button.
The display returns to the [TWAIN Driver] dialog box.
Continue scanning from this dialog box.
62 4.6 Detecting Double-feeds
: The scanner uses a sensor to monitor the
length of documents that are fed. It
detects a double-feed by changes in the
document length when two or more
document sheets are fed overlapping.
: The scanner monitors both document
thickness and length to detect double-
feed.