Prince George was the second Duke of Edinburgh, before he became George III.
The title was first created in the Peerage of Great Britain on 26 July 1726 by George I, who bestowed it on his grandson Prince Frederick, who also became Prince of Wales the following year. The subsidiary titles of the dukedom were Baron of Snowdon, in the County of Caernarvon, Viscount of Launceston, in the County of Cornwall, Earl of Eltham, in the County of Kent, and Marquess of the Isle of Ely. These titles were also in the Peerage of Great Britain. The marquessate was apparently erroneously gazetted as Marquess of the Isle of Wight although Marquess of the Isle of Ely was the intended title. In later editions of the London Gazette the Duke is referred to as the Marquess of the Isle of Ely. Upon Frederick's death, the titles were inherited by his son Prince George. When Prince George became King George III in 1760, the titles "merged into the crown", and ceased to exist.
Forty years later, on 19 November 1764, George III created a variation of the title for his younger brother, Prince William, the full form being "Duke of Gloucester and Edinburgh". This title was in the Peerage of Great Britain. The subsidiary title of the dukedom was Earl of Connaught, in the Kingdom of Ireland, a title in the Peerage of Ireland. In 1805, the titles were inherited by William's only son, Prince William Fredrick, who died without a male heir, causing the title to cease to exist.
The title was created for a fourth time on 19 November 1947 by King George VI, who bestowed it on his son-in-law Lieutenant Philip Mountbatten, when he married Princess Elizabeth. Subsequently, Elizabeth was primarily known as HRH Princess Elizabeth, Duchess of Edinburgh until she became Queen in 1952. The subsidiary titles of the dukedom were Earl of Merioneth and Baron Greenwich, of Greenwich in the County of London. Like the dukedom, these titles were also in the Peerage of the United Kingdom. Earlier that year, Philip had renounced his Greek and Danish royal titles (he was born a Prince of Greece and Denmark, being a male-line grandson of King George I of Greece and male-line great-grandson of King Christian IX of Denmark) along with his rights to the Greek throne. In 1957, Philip became a Prince of the United Kingdom.
It was announced that in 1999, at the time of the wedding of Prince Edward, Earl of Wessex, he would follow his father as Duke of Edinburgh. This is unlikely to happen by direct inheritance, as Prince Edward is the youngest of Prince Philip's three sons. Rather, the title is expected to be newly created for Prince Edward after it "eventually reverts to the crown" after "both the death of the current Duke of Edinburgh and the Prince of Wales' succession as King."