[megaloman]

CMD (дублирует строки прямо в оригинальном файле):

Код:

@Echo Off
cls
	Set "FileIn=Z:\Box_In\txt.txt"
	Set /A N=5
	
	Set /A Rep=10000000
	FOR /F "usebackq delims=" %%s IN ("%FileIn%") DO Set /A Rep+=1 &Call Set "@@%%Rep%%=%%s"
	>"%FileIn%" (FOR /F "usebackq tokens=1* delims==" %%a IN (`2^>nul Set "@@"`) DO (
		FOR /L %%i IN (1,1,%N%) DO (Echo %%b)
	))
Pause
Exit /B