It would help if you gave information about the files such as the name, file size and file type.
Every day I find up to about 10 of the files in my downloads folder are duplicated, sometimes 2 or 3 times. I delete all the duplicates and even some of the originals then next day there is another lot often accompanied by files named with a single letter or a long name of apparently random letters.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic