Before 1752, the English government still observed March 25 as the first of the year, despite most of the population observing January 1. This was the "old" Julian calendar.
The practice of double dates resulted from the switch to the Gregorian calender.
For this reason, many people wrote dates falling between January 1 and March 25 with both years. For example, 20 March 1718 in the "old" Julian calender was 20 March 1719 in the "new" Gregorian, so to avoid confusion "double dating" was often used which in this case would be 20 March 1718/19. The British government decreed that 2 September 1752 be followed by 14 September 1752 to put England in line with the Gregorian calender.