How to read line by line and match text with next line?

Question

read the files data line by line and match the line with the next line, if it matches then save that line in the uniqe.txt file.

Answer 1

You can accomplish this with a simple for-loop

#!/bin/bash

mapfile -t < text.txt
for ((a=0,b=1; $b<${#MAPFILE[@]}; a++,b++)); do
     [[ ${MAPFILE[$a]} = ${MAPFILE[$b]} ]] && echo ${MAPFILE[$a]}
done > unique.txt

Answer 2

Not sure if "successive duplicate lines" is a key issue for you. If not then you simply need the Linux command uniq to eliminate duplicate lines in the file with:

uniq -u inputfile.txt > uniqe.txt

If, however, you are only interested in eliminating successive duplicates you can use awk:

awk 'NR == 1 {a=$0; print}  a!=$0 {a=$0; print}' inputfile > uniqe.txt