1) save it to file, say, test.txt
2) cat test.txt test.txt >> /var/spool/mail/$USER
  (more than one mail is important!)
3) receive the mail with gnus (i used nnmbox backend)
4) mark both messages using hash (#)
5) press C-M-d

then the summary buffer for these digests looks like:

   [   1: aa1@bbb.ccc         ] subj1
       <   1: aa1@bbb.ccc         > 
   [   1: Gr\201ü\201ß Gott         ] Gr\201ü\201ß Gott
       <   1: Gr\201ü\201ß Gott         > 
   [   1: aa3@bbb.ccc         ] subj3
       <   1: aa3@bbb.ccc         > 

something is wrong with handling of multibyte buffers in gnus.
such problems appear quite regularly... :-(

	Best regards, -- Vladimir.