Formula to assign an Item number for values in another column

Hi,

I have a list of package numbers down column B in my spreadsheet. The numbers are of the format 70, 120, 150, 220, etc.

There are multiple entries for each of these package numbers and I want to put a formula in column A to assign an ordered “Item” number to each package number.

For example, for package number 120 I want 1 to appear in column A the first time 120 appears in column B, 2 for the next, etc. For package 220 I want 1 in column A for the first instance in column B, 2 for the second, etc.

I have attached an example spreadsheet where I have manually filled in the first few Item numbers in column A, as an example of what I need.

What formula do I need in column A?

Thanks very much

Dave