Excel Multiple Duplicates Conditional Formats

Hey there, I have been racking my limited knowledge of excel trying to work this out but cant get it right! I have searched and searched but just cant solve it!

I have a list of phone numbers on sheet1 and another list of phone numbers in sheet 2. What I want is to make the excel sheet highlight any duplicates across the various sheets. (I am expecting through time I will have more sheets with numbers on).

I have done the formula =COUNTIF($A$1:$A$100,A1)>1 and that works for highlighting the duplicates on that sheet. But it wont do cross sheets. I have tried this as well: =OR(COUNTIF(sheet1,A1)=1,COUNTIF(sheet2,A1)=1) but that didnt seem to work.

Any advice, in a broken down idiots guide, would be greatly appreciated!

I have read a previous post here but that was the above formula and just wont work for me! :-(

Try

=OR(COUNTIF($A:$A,A1)>1,COUNTIF(Sheet2!$A$A,A1)>1)