@echo off
set /a m=2
(for /f "delims=" %%a in (a.txt) do (
    set /a "n+=1,1/(n%%m)" 2>nul||echo %%a
))>b.txt
pause
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@echo off
 setlocal enabledelayedexpansion
set n=0
for /f "delims=" %%a in ( t6.txt ) do (

  set /a n+=1 
 set /a row=!n!%%3
  if !row! equ 0 echo %%a
)

pause
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sed -n "n;p" a.txt
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