Link worksheets in Excel

I have a spreadsheet with multiple worksheets. I want to run conditional formatting/highlight cell rules/duplicate values to mark duplicate data (item numbers). The worksheets are labeled by month and there are 12 of them. For instance, if on sheet 1 (january) in cell B617 the number is 23466778 I want to see if there is the same number on any of the worksheets. I use ctrl f, then find all, and so far it will only check the active worksheet.