@echo off
REM 缺点1：无法处理特别大的文件
REM 缺点2：需要使用文件中没有出现的字符来标记变量（本例中使用的是下划线）
setlocal
for /f "delims=" %%i in (a.txt) do (
  set /a _%%i+=1
)
(for /f "tokens=1-2 delims=_=" %%i in ('set _') do (
  if %%j gtr 1 (
    echo,%%i
  )
))>b.txt
endlocal

@echo off
setlocal enabledelayedexpansion
set "PriLine="
set "DupNum=1"
(for /f "delims=" %%i in ('sort a.txt') do (
  if "!PriLine!" equ "%%i" (
    set /a DupNum+=1
  ) else (
    if !DupNum! gtr 1 (
      echo,!PriLine!
    )
    set DupNum=1
  )
  set "PriLine=%%i"
))>b.txt
if !DupNum! gtr 1 (
  >>b.txt echo,!PriLine!
)

gawk "{a[$0]++}END{for(i in a)if(a[i]>1)print i}" a.txt > b.txt

gawk "a[$0]++" a.txt>b.txt

@echo off
for /f "tokens=*" %%i in (a.txt) do set #%%i=%%i
(for /f "tokens=2 delims==" %%i in ('set #') do echo %%i)>b.txt