Groupby sum duplicates rows

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,

Hi @Karen2022

Welcome to the KNIME community!

Could you please include some (anonymized) examples of your data set, what the current output is and what the expected output is? This way people are better able to help you out. Right now it’s a bit hard to visualize what you’re encountering.