Windows – how to set $HOME variable so that cmd find the path

Similar to MaddHackers answer, just more compact.
echo %path%|find /i "%np%">nul || set path=%path%;%np%

%np% is your new path, of course you can use literals instead. What it does: echo %path%|find /i "%np%">nul searches existing path for a string, discarding output. || means execute on failure, so it means: Search path for string to be added, and if not found, add it.

Edit: Generally it's not required to quote paths, even those containig spaces, but if you do want to quote them, this version will work with double quoted paths:
echo %path%|find /i "%np:"=%">nul || set path=%path%;%np%

Edit: changed findstr /i /c: to find /i as findstr may misinterpret some sequences as noted by KubaOber in comments

This can be a problem when you have a very long PATH variable and/or a huge amount of other long environment variables set. It can also happen if you have spaces next to your semicolon delimiters and if you have a % as the first character in your path.

You may have some luck with FixPath which fixes some common path problems automatically.