How to edit/delete identical/duplicate text lines common to multiple .txt files at once?

Let's say I have multiple lists of quotes. These lists represent the categories of quotes. Some quotes can be present in multiple categories (lists).

I need to be able to read and edit the lists manually. Best for this purpose is .txt files but I would not know if the quote I am looking at is also present in other .txt file. So if I edit the quote or delete the quote, it won't be deleted from other .txt files.

What is the solution?
(If I choose to edit or delete the quote and if it is present in other .txt file/s as well, i need it to get edited or deleted automatically)

If you really want to stick with this structure, the best you could probably do would be select the quote and run a macro that will tell you which files it is it and then you can deal with the results.

But really the better solution is to change the way you are doing this. Perhaps have a single file with all the quotes, each with a list a categories. Then use a command to extract all the quotes for specific category as needed.

Duplicating the quotes is always going to cause you problems.