identifying duplicates

Hello everybody,

Here's my problem. I have a huge (25,000 row) spreadsheet with two 'ordered' columns of 6 digit co-ordinate data (eastings and northings). I now want to create a third column which identifies duplicates.

Essentially, what I want to do is each time column-A and column-B are duplicated in a series of rows, then create unique flag in the third column...

e.g.

123456, 989876, 1
123456, 989876, 1
765432, 343232, 2
456543, 686843, 3
456543, 686843, 3
456543, 686843, 3
675423, 776453, 4
282864, 966572, 5
282864, 966572, 5
282864, 966572, 5
...etc

Any help much appreciated,
Many thanks,
Helen

Thanks Dave,
You're my hero xxx