Repeating data items across years

Hi all,

I have a dataset where some items occur more than once. I would like to calculate the rate of repetition. The trick is that it's across years. I've created a new variable which indicates whether a given row has the same ID as another row and gives it a higher number. So, for example, this variable will be 1 for any item that doesn't have more than one row, but for an item with several rows, its first or index occurrence will be 1 and its forty-second occurrence will be 42. By simple maths I can work out how many rows are >1 for this row and then the repetition rate.

But where I'm stumped is: how do I find out how many 2001-indexed items recur in any year?

Thanks!