Canon 3301B002 On-Screen Manual page 448

Scanning Both Sides of Documents at One Time Page 448 of 997 pages
Scan Settings Dialog Box (Stack of Documents (ADF))
5.
Return to the Scan/Import window and click Scan.
The back side of each document is scanned automatically after scanning the front side.
When scanning is completed, the Scan Complete dialog box opens. Select Scan or Exit. Select
Scan to scan the next document, or select Exit to end.
The scanned images appear in the Thumbnail window.