@echo off
setlocal enabledelayedexpansion
set newline=^& echo.
set final=
for /f "tokens=*" %%_ in ('dir /b') do set "final=!final!%%_!newline!!newline!"
echo %final%
 

@echo off
setlocal enabledelayedexpansion
for /f %%_ in (a.txt) do (type "b.txt" | findstr /r "^%%_$" > nul 2>&1 && (for /f "tokens=*" %%# in ('type "b.txt" ^| findstr /r /i "^%%_$" ^| find /v /c ""') do (if %%# equ 1 (echo %%_ >> final.txt))))

C:\Users\leo\batch>type a.txt
1
1
2
3
4
4
5
6

C:\Users\leo\batch>type b.txt
1
1
2
3
4
4
5
6

C:\Users\leo\batch>code

C:\Users\leo\batch>type final.txt
2
3
5
6

C:\Users\leo\batch>