Sudoku Problem

I have a problem with this Sudoku puzzle. It is labeled medium difficulty, which sometimes leads to doing them too quickly. The rule is that there is only one solution allowed. So, assuming that the puzzle setter is playing by the rules, what is the mistake that I made so that when I get down to two or three couples (as you can see in the middle column: 18 in the top box, 16 in the middle, and 68 in the bottom box) and the numbers appear to be interchangeable?

This has happened once or twice before and when I looked at the answer the next day my numbers all looked like gobbledegook, so I expect that will happen again, but the irritating thing is that looking at the answer doesn’t tell where or when you went wrong.

I know this is embarrassing, but that is one of the advantages of advancing age: Embarrassment doesn’t bother one so much anymore. Can anyone help, either with the answer to my question or directions to where I might find the answer?

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