How to display the names of the columns of a csv file in which there is a duplicate cell value?

Question

Python

python 3, pandas latest version.
There is a csv file.
Strings - product url.
Columns - product categories. (That is, one product can be in several categories, as often happens)
Tell me the optimal (or any) algorithm that for each cell will return a list of column names where its duplicate occurs.
Thank you!

Answer 1

Rotate the table so that each row has a pair of (url, category (which used to be a column name)).
Then search for duplicate rows.