Highlighting duplicate data based on two conditions

Ah. Ok. My apologies. I forgot that you needed semicolons. Try changing it and see if that corrects the issue.

Never be shy about asking for help. All that does is make it even longer before you can get some help. No judgement here.

New Request: I am still not sure I follow. You want a formula on all 10 sheets to tell you if an ID is on any of the other 9?

You won't necessarily have to remove it. You could either add to it so that the same flag appears for dups on the same sheet or the others, you could add another flag column to differentiate between same sheet vs others, or you could even add in a column for each sheet to flag. The main difference is going to be how many columns you want to use.

Here's the basics...

=CONTAINS([ID#], {Sheet 2 ID# Column})

Since CONTAINS is a true/false, it will flag if true or leave blank if false. Using this logic, you can create a nested IF to have one column flagged if it is found in one or more other sheets, drop the formula as-is into it's own column (one column for each sheet) to flag which sheet(s) you are looking for, or add the nested IF to your current dup formula to have same sheet and other sheets all flagged in the same column.

Nested IF example:

=IF(CONTAINS([ID#], {Sheet 2 ID# Column}) = true, 1, IF(CONTAINS([ID#], {Sheet 3 ID# Column}) = true, 1, IF(CONTAINS([ID#], {Sheet 4 ID# Column}) = true, 1, ..........................................................

How did you create your cross sheet reference? I notice a space between "ID#" and the closing curly bracket. Does that belong there?