xxpinqz

@echo off&setlocal enabledelayedexpansion
(for /f "tokens=1-2 delims==" %%a in ('sort a.txt') do (
    if not "!str!"=="%%a" (
    	if defined n echo,!str!=!n!
    	set "n="
    )
   set/an+=%%b
   set "str=%%a" 
)
echo,!str!=!n!)>b.txt
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