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WINDSOR, UNITED KINGDOM - MAY 01: Princess Sophie of Hanover, Prince Philip's sister watching him compete at the Royal Windsor Horse Show in the 1980s. (Photo by Tim Graham/Getty Images)

WINDSOR, UNITED KINGDOM - MAY 01:  Princess Sophie of Hanover, Prince Philip's sister watching him compete at the Royal Windsor Horse Show in the 1980s.  (Photo by Tim Graham/Getty Images)