From Just Solve the File Format Problem
"DUO" is a set of similar graphics formats for the Atari computers with unknown origin. The original name, if anything, is unknown.
Files are exactly 113600 bytes long.
From the AtariForumWiki page:
DUO *.DU1 *.DUO (low resolution) *.DU2 (medium resolution) Created by Anders Eriksson: http://ae.dhs.nu/ These are essentially 2 screens with over-scan interlaced together. Low resolution files are 416x273 pixels: 16 words palette 56784 bytes screen 1 56784 bytes screen 2 ----------- 113600 bytes total Medium resolution files are 832x273 pixels: 4 words palette 56784 bytes screen 1 56784 bytes screen 2 24 bytes padding if present, otherwise total size is 113576 bytes ----------- 113600 bytes total Both screens use the same palette, but when viewed the human eye merges the two alternating images into one.