Detect duplicates

Can somebody help me with this quite complicated problem. I need to detect duplicate row in excel, i cant use "Remove duplicate" function because I want to see what are duplicates.

*Every row has got 4 cells.
*I have got around 50k rows which have do be process
*Those rows which are duplicates must be marked with numbers 1 2 3 etc.
*Complicated problem: Column A and B has got coordinates which may vary around +- 0.001

There a picture to imagine what i need:
3 rows.png

Ask questions, maybe i didn't explained the problem clearly.