This error indicates that the specific conversion that is being uploaded to your Google Ads account was already uploaded.

What could cause this for example is if you export all conversions for the dates 01-09-2021 to 07-09-2021 and then at a later date exporting the conversions for one of the days that were already uploaded. For example if you then exported the conversions from 07-09-2021 to 14-09-2021 then the data for the day 07-09-2021 will have already been uploaded.

It is possible to prevent this issue by ensuring that you only export conversion data for dates that were not previously exported.