$ cat demo.txt
redhat
debian
ubuntu
ubuntu
centos
fedora
fedora
fedora
fedora

To report or filter out for lines that are adjacent and repeated

$ uniq demo.txt

To only print unique lines

$ uniq -u demo.txt

To prefix lines by the number of occurrences

$ uniq -c demo.txt

To only print duplicate lines, one for each group

$ uniq -d demo.txt

To print all duplicate lines 

$ uniq -D demo.txt

To print all duplicate lines but  allow  separating  groups  with  an  empty line

$ uniq --all-repeated=prepend demo.txt

To ignore differences in case when comparing

$ uniq -i demo.txt

To skip comparing the first X number of characters in each line

$ uniq -s 3 demo.txt

To compare only the first X number of characters

$ uniq -w 4 demo.txt

regards,

T.Dhanasekar