Download icon appears next to songs in iTunes, but files already downloaded

I have my iTunes library stored on a NAS drive and am using the iTunes app on an iMac.

Most of my songs in iTunes have the download icon next to them, but they are already downloaded on the NAS drive.

If I click to download, then a second version of the file is downloaded to the same location (with 1 added to the file name).

Is there a way to tell iTunes that the files are all already there? I don't really want to have to download them all again.

Thanks

deb