Ought puzzles (like sudoku) have only 1 solution?

[Thudlow_Boink]

Recent thread on the same topic:

Sudokus don't always have unique solutions

Yes, I may be a few decades late to this realisation. I was doing a sudoku on a flight and got horribly stuck towards the end. All of the remaining cells only had one of two values that they could be, but there didn’t seem to be any logical basis to rule out either value for any cell. After staring at it for about half an hour, I eventually realised that both sets of values made complete and correct sudoku grids. I had always assumed sudokus were always solvable to one unique solution.