counting duplicates in a sorted sequence using command line tools

how about;

$ echo "100 100 100 99 99 26 25 24 24" \
    | tr " " "\n" \
    | sort \
    | uniq -c \
    | sort -k2nr \
    | awk '{printf("%s\t%s\n",$2,$1)}END{print}'

The result is :

100 3
99  2
26  1
25  1
24  2

uniq -c works for GNU uniq 8.23 at least, and does exactly what you want (assuming sorted input).

Numerically sort the numbers in reverse, then count the duplicates, then swap the left and the right words. Align into columns.

printf '%d\n' 100 99 26 25 100 24 100 24 99 \
   | sort -nr | uniq -c | awk '{printf "%-8s%s\n", $2, $1}'

100     3
99      2
26      1
25      1
24      2

if order is not important

# echo "100 100 100 99 99 26 25 24 24" | awk '{for(i=1;i<=NF;i++)a[$i]++}END{for(o in a) printf "%s %s ",o,a[o]}'
26 1 100 3 99 2 24 2 25 1