As I pointed out earlier, the 2016 one was the second referendum on whether we should leave.
It should be pointed out that:On Thursday 5 June 1975 the United Kingdom held its first ever nationwide referendum on whether to continue its membership of the European Communities (EC) principally the European Economic Community (EEC, or "Common Market") as it was more widely known at the time.
This is a very different entity to what exists today as the EU.
'By the Maastricht Treaty (formally known as the Treaty on European Union; 1991), which went into force on November 1, 1993, the European Economic Community was renamed the European Community and was embedded into the EU as the first of its three “pillars” (the second being a common foreign and security policy and the third being police and judicial cooperation in criminal matters). The treaty also provided the foundation for an economic and monetary union, which included the creation of a single currency, the euro. The Lisbon Treaty, ratified in November 2009, extensively amended the governing documents of the EU. With the treaty’s entry into force on Dec. 1, 2009, the name European Community as well as the “pillars” concept were eliminated'

You cannot compare apples to oranges. There is a vast gulf between the old EEC and the morphed EU of today.
The tail now wags the dog! That is why the last referendum cannot be considered the second referendum.