The letters a e i o r u and their Cyrillic equivalents а е и о р у can all be found with the double grave accent. Unicode provides precomposed characters for the uppercase and the lowercase Latin letters but not the Cyrillic letters.
Unicode provides pre-composed characters for both uppercase and lowercase Latin letters, but not for the Cyrillic versions of a e i o r u. The double grave accent is used to represent these letters in Unicode.