I finally found this answer which tells how to use data validation to make sure all the values in one column are unique.
But it doesn't tell if there is a way to write a single validation rule that can make sure that all the values in multiple columns are unique per column. So far the only way I see is to replicate the validation rule for each (of 50 or so) columns individually.
Is there a way to do that with a single rule? It would hinge on where to put the $, I suppose, and the rules for doing that don't seem to be well documented, especially how they get interpreted inside validation rules.
Best Answer
Well, after a bit of fiddling, I found a solution that currently works:
Specify the cell range as desired: for the whole columns, starting with column A and ending with column BB, so it is
Then the custom formula should be:
It seems like the Data Validation operation the places this "validation formula" (which must be True to avoid a warning) in every cell, as if a regular formula were copy/pasted to the cell. Which might allow a bit of intuition as to when and how to place the $ in the cell references, if true. It seems as if the formula should be written as if for the top left cell of the range specified.