COUNTIF Type Mismatch Error When Preventing Duplicates via VBA

Hey everybody,

I am currently building an inventory spreadsheet in which I am keeping track of a bunch of serial numbers.

I have a VBA code I am using to prevent duplicate number entries within a specific range.

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It works flawlessly... Except I get a "Type Mismatch" error when I try to change/delete multiple cells, or when I try certain formatting like inserting a row. This happens regardless of whether I make the change in the given range or not.

Any help is much appreciated

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