terse

这样呢

@echo off&setlocal enableDelayedExpansion
(for /f "delims=" %%i in (a.txt) do (
    if not defined _"%%i" (
        echo,%%i
        set _"%%i"=%%i
    ) else if not "!_"%%i"!"  == "%%i"  echo %%i
))>b.txtCOPY

没测试

@echo off&setlocal enableDelayedExpansion
(for /f "delims=" %%i in (a.txt) do (
        if not "!_"%%i"!"  == "%%i"  echo %%i
        set _"%%i"=%%i
))>b.txtCOPY

terse

@echo off
powershell "$Hash = @{};(GC '.\a.txt').Where({if (!$Hash.Contains($_)){$Hash.Add($_,++$i)}});($Hash.GetEnumerator()| Sort Value).Key"
pauseCOPY