The only way a second referendum can overturn the first, ultimately, is if the electorate as a whole changes its mind and votes the other way. I can certainly sympathise with the idea that offering the referendum in the first place is either cynical or undemocratic, but none of that will matter once the vote itself takes place -- assuming, of course, that you trust the electorate to reach a sensible and informed decision.
Since the 2016 referendum was also held for cynical, undemocratic reasons -- it was Cameron's failed attempt to unite his party and quash Euroscepticism; the idea that the public might actually vote Leave clearly never realistically occurred to him, otherwise he'd have done far more to prepare the country for that outcome -- since it was also held, as I say, on cynical grounds, there's no particular reason to be especially critical of the purpose behind a possible second one.