I don’t remember anyone before the referendum saying it would be the first of two.
A lot of people pushed unsuccessfully for a second referendum after the first one but that was never based on any pre-existing legalities or precedent, it was just an attempt to overturn the first result from people who were upset that theyd lost.
In hindsight, I think I was misremembering that a second referendum was debated beforehand. But the non-binding aspect of the first had me thinking it would lead to further definition before it was embarked upon. Was there elections held between referendum and determining to do it?
Also based on the fact that no specific plan for brexit had popular support.