Does not take uploaded documents after a certain date into account

[della66]

Every month I need to calculate our dealers' commissions.
In order for a dealer to be commissioned, we need to physically have the contract the customer signed, so before I run the commission tool, I upload the received contracts into my "dealer database/commission tool".
Last month, some contracts were missing from the file I use to upload the received contracts. When I noticed this, I just entered them into the file I have to upload into the DB, and I uploaded the "received contracts" file into the Dealer DB/Commissioning tool" for a second time that month. It did not work, but since this only concerned a few contracts, I didn't bother too much...
Now, this month I uploaded the received contracts again (and also the contracts received since then), but since I uploaded received contracts twice in the same month, the DB only takes contracts from two months ago into account when calculating the commissions, and neglects the ones from the last month.... I'm looking for a way to fix this issue, but no success, so I taught I might ask you guys...

[orange]

It appears you have several tools on the go. You are receiving contract information in some format from someone/somewhere; you are uploading the received file(s) into the
Dealer DB/Commissioning Tool; you uploaded some contracts twice in the same month.

You have some procedural issues in that you do uploads without checking for duplicates.
It also seems you receive a file(s) that are likely not validated in any way.

Which part exactly is the database?
What checks and balances are in the Data base to prevent duplicates/validate the incoming data?
What is in your manual procedures to prevent duplicates and/or verify all the correct Contracts have been included?
What are the business rules associated with processing the Contracts?

[June7]

We don't know what criteria you are using to generate the commission calculation. Do you input a date value as a cut-off to 'include all contracts received after given date'? Or do you have yes/no or date field that is populated as indicator 'this record already included in commission calc' and use that field to filter commission report?