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KING'S LYNN, ENGLAND - FEBRUARY 04: Queen Elizabeth II and Prince Philip, Duke of Edinburgh attend Sunday Service at St Peter and St Paul Church in West Newton on February 4, 2018 in King's Lynn, United Kingdom. (Photo by Mark Cuthbert/UK Press via Getty Images)

KING'S LYNN, ENGLAND - FEBRUARY 04:  Queen Elizabeth II and Prince Philip, Duke of Edinburgh attend Sunday Service at St Peter and St Paul Church in West Newton on February 4, 2018 in King's Lynn, United Kingdom.  (Photo by Mark Cuthbert/UK Press via Getty Images)

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EDINBURGH, UNITED KINGDOM - JULY 04: Prince Philip, Duke of Edinburgh smiles during a visit to the headquarters of the Royal Auxiliary Air Force's (RAuxAF) 603 Squadron on July 4, 2015 in Edinburgh, Scotland. (Photo by Danny Lawson - WPA Pool/Getty Images)

EDINBURGH, UNITED KINGDOM - JULY 04: Prince Philip, Duke of Edinburgh smiles during a visit to the headquarters of the Royal Auxiliary Air Force's (RAuxAF) 603 Squadron on July 4, 2015 in Edinburgh, Scotland. (Photo by Danny Lawson - WPA Pool/Getty Images)

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LONDON, ENGLAND - JUNE 04: Queen Elizabeth II sits with Prince Philip, Duke of Edinburgh as she delivers her speech during the State Opening of Parliament in the House of Lords at the Palace of Westminster on June 4, 2014 in London, England. Queen Elizabeth II is to unveil the coalition government's legislative programme in a speech delivered to Members of Parliament and Peers in The House of Lords. Proposed legislation is expected to be introduced on a 5p charge for plastic bags in England, funding of workplace pensions, new state-funded childcare subsidy and reforms to speed up infrastructure projects. (Photo by Ray Collins - WPA Pool/Getty Images)

LONDON, ENGLAND - JUNE 04:  Queen Elizabeth II sits with Prince Philip, Duke of Edinburgh as she delivers her speech during the State Opening of Parliament in the House of Lords at the Palace of Westminster on June 4, 2014 in London, England. Queen Elizabeth II is to unveil the coalition government's legislative programme in a speech delivered to Members of Parliament and Peers in The House of Lords. Proposed legislation is expected to be introduced on a 5p charge for plastic bags in England, funding of workplace pensions, new state-funded childcare subsidy and reforms to speed up infrastructure projects.  (Photo by Ray Collins - WPA Pool/Getty Images)

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LONDON, ENGLAND - JUNE 8: Prince Philip, Duke of Edinburgh, smiles after unveiling a plaque at the end of his visit to Richmond Adult Community College in Richmond on June 8, 2015 in London, England. Prince Philip, officially opened and was shown round the new art, drama and dance facilities at the further education college which offers up to 2,000 courses. (Photo by Matt Dunham - WPA Pool / Getty Images)

LONDON, ENGLAND - JUNE 8: Prince Philip, Duke of Edinburgh, smiles after unveiling a plaque at the end of his visit to Richmond Adult Community College in Richmond on June 8, 2015 in London, England.  Prince Philip, officially opened and was shown round the new art, drama and dance facilities at the further education college which offers up to 2,000 courses.  (Photo by Matt Dunham - WPA Pool / Getty Images)

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WINDSOR, ENGLAND - APRIL 11: Britain's Queen Elizabeth II and Prince Philip, Duke of Edinburgh react as they bid farewell to Irish President Michael D. Higgins and his wife Sabina (not pictured) at the end of their official visit at Windsor Castle on April 11, 2014 in Windsor, United Kingdom. Ireland's Michael D. Higgins is making the first state visit by a president of the republic since it gained independence from neighbouring Britain. The visit comes three years after Queen Elizabeth II made a groundbreaking trip to the republic, which helped to heal deep-rooted unease and put British-Irish relations on a new footing. (Photo by Leon Neal-WPA Pool/Getty Images)

WINDSOR, ENGLAND - APRIL 11:  Britain's Queen Elizabeth II and Prince Philip, Duke of Edinburgh react as they bid farewell to Irish President Michael D. Higgins and his wife Sabina (not pictured) at the end of their official visit at Windsor Castle on April 11, 2014 in Windsor, United Kingdom. Ireland's Michael D. Higgins is making the first state visit by a president of the republic since it gained independence from neighbouring Britain. The visit comes three years after Queen Elizabeth II made a groundbreaking trip to the republic, which helped to heal deep-rooted unease and put British-Irish relations on a new footing.  (Photo by Leon Neal-WPA Pool/Getty Images)