I realize my orginal question was unclear ! I would need the formula to check if the same record appears in both columns, not if any record appears in both columns.
Told differently, those are questions for a form. I need to check if any answer given to question 1 was the same as the answer to question 2.
So if question 1 has [1,2,3] and question 2 has [2,5,6] it should return true.
To solve that problem, create that formula in the OTHER table instead of the CURRENT table.
Then, back in the CURRENT table, create a rollup field that points to the formula field with this formula:
And be sure to set the condition to only show you values when the formula field is 1.