Adding a new dataset doubles the values

Hello everyone,

I've set up a PowerbI report based on data I receive monthly. I've created a Sharepoint folder which is where the data set is located.

I receive a monthly YTD file, which includes data from january 1st until the end of the month of the month I receive a new file. 
My goal is to upload the new YTD file to the same Sharepoint folder, so that the report includes the latest month of data. 

However, when I upload the new file, my values seem to double in PowerBI. 

This is before:

This is after:

It just seems to add the values in the new file, with the ones in the previous file, but I only want it to add the values of the latest month.

Any help would be greatly appreciated