How to quickly highlight duplicate cell values across entire workbook (all sheets)

Hi.
I'm working on a job where every day I receive a list of files.
To track the progress of each file, I've set up an Excel file. Each day has its own sheet (aka tab). Each day when the files come in, I copy the file names for that particular day into a column in the sheet created for that day.

However, I've noticed that I occasionally get duplicate files. For example today I received 36 files, of which seven, I noticed, had been sent to me on previous days. And the work on those files is already complete, meaning I can save myself time if I could somehow be made aware of duplicates, as they come in each day and avoid doing the work multiple times for the same file.

Is there some way I can quickly highlight cells containing the same file names?

In theory, I thought I could select tab of first sheet, and with SHIFT held down select last tab of last sheet, select all (ctrl+A), and then select Home > Conditional Formatting > Highlight Cell Rules > Duplicate Values.

But when I select multiple sheets (select tab of first sheet, and with SHIFT held down select last tab of last sheet) the Conditional Formatting button becomes greyed out and unavailable.

Am I doing something wrong? Is there a better way to approach this?

Mind you I'm using Excel 2013 (my employer is too cheap to update to a newer Office)

Instead of each day having it's own sheet, are you able to put all days on the same sheet, but give each day it's own column? Conditional formatting should then work.

Hi OpieWinston,
Technically it's possible to have it all in one huge sheet, but very inconvenient. I'm using many columns on each sheet as part of the progress tracker, so I'd have to do a lot of scrolling left and right.
Clicking through tabs is just easier to deal with from a UX perspective.
From your reply, should I assume that what I'm hoping to do is not possible (at least not done easily)?