The British monarch celebrates two birthdays, one on their actual birthday and then officially in the summer when the weather is nice enough for a parade. – WTF Fun Facts
Source: https://www.bbc.co.uk/newsround/36489213
Fun Fact – British Monarchs Birthdays
What if the birthday is actually in the summer? In such a case I guess it will be only one time. The phrasing should change in my opinion.