Linux command or script counting duplicated lines in a text file?

Almost the same as borribles' but if you add the d param to uniq it only shows duplicates.

sort filename | uniq -cd | sort -nr

uniq -c file

and in case the file is not sorted already:

sort file | uniq -c

Send it through sort (to put adjacent items together) then uniq -c to give counts, i.e.:

sort filename | uniq -c

and to get that list in sorted order (by frequency) you can

sort filename | uniq -c | sort -nr