uniq
TextPackage: coreutils
Removes adjacent repeated lines, or with -c counts them. It sees only neighbours: sort | uniq -c is the pair, and uniq alone on an unsorted file removes nothing.
What its options do in the lessons
From the same glossary the lessons render under their commands, so the two cannot disagree.
uniq -c- Prefixes each line with the number of times it occurred.
uniqonly groups adjacent lines, so it goes after asort.