How to turn off export to * .eml already exported emails?

When re-exporting mail folders to * .eml, previously exported mails are copied again and a string with the number: “(1) … (x)” is added to the file name according to the number of exports. So I have more files with the same content in the folder.

Is it possible to set somewhere that the export is performed only for new mails that are not yet in the folder with exported * .eml (without already exported mails) ???

No, that is not possible, but in Explorer you can just search the folder afterwards for any (1) and delete the results.