From her birth in 1897 to the death of Queen Victoria on 22 January 1901 Princess Mary was a "mere" HH. Did this lesser style apply only to females, or were her brothers Princes Albert and Henry also HHs for a short time, while the eldest son, Edward, was an HRH?
We are familiar with the Letters Patent of 1917 stipulating who holds the HRH, but what was the situation prior to that revision? Edward VII in 1905 raised his granddaughters Ladies Alexandra and Maud Duff to the rank of Princess with the style of Highness, and Prince Alastair of Connaught was an HH Prince of Great Britain and Ireland from his birth in 1914 until 1917. The only other cases I can recall of the use of HH (albeit after George V's Letters Patent) are the Princesses Helena Victoria and Marie Louise who relinquished their German titles of Schleswig-Holstein and Anhalt respectively in 1917 to become HH Princesses [of nothing].