I need some idea about why Groupby/ sum, is duplicating rows and adding twice.
I have a small data with 35,000 rows, with invoice numbers and quantity sold. I
fix the data to integrate some classifications like month and year. So, if I grouping by month and year and sum including invoice number, it gives me a result, but if I add the data without grouping, it gives a lower value.
I exported both data and found that some lines were being duplicated and added twice.
I wonder why GB duplicates some rows,