Highlight duplicates data form sheet to another sheet

[bixha]

Basically I have a spreadsheet which shows months Sep ,Oct, Nov and Dec we have to tracking ID number so if ID number is sheet Sep or Oct I want to highlight it as duplicates data so that case I am informed that ID number has been last month. What I am trying to do is highlight duplicate ID Numbers across the worksheets as we shouldn't have the same ID Number in the two different sources.
See attachment.

[vichopalacios]

Checking your file, there are a lot a duplicated values in every filled tab....

So, as per your example:
you can have duplicated numbers in the same tab, but you want to highlight if the number you are entering is already in any previous tab?

[bixha]

I have numbers in two sheets in the same excel workbook. I need to compare the data in Column C:AI, Sheet (Sep) with the data in C:AI, sheet (Oct) and highlight the duplicates.

[vichopalacios]

Check the attached file..:

I've placed in tab "Oct" a formula in conditional formating for the table area, and only 4 values are not repeated in tab "sep"
take care of the new table I made down the original. It is used only for showing how it works, not needed for the formating.

[bixha]

thank you