There can be multiple original press photos. An original press photo is a photo made directly from the original negative soon after the image was shot. Ordinarily, there will be only several at most of a particular original press photo, as a limited number were made and many are lost or destroyed. If you have a 1920 original of Ty Cobb there may only be 5 in existence, maybe only one. There won't be 100 of them. The photos were only distributed to subscribing newspapers and magazines, so there is a finite limit to the number.
You can usually determine the age of a press photo by the stamping and tag on back. For example a Pacific & Atlantic stamp should show the photo is old, as that photo service existed only a short time. Then, if the image is crystal clear that would show it was made from the original negative.
The below linked pdf guide includes a chapter on press photos, on the stampings and tags. It has a list of the stamps and when they date from. This alone will serve you well in collecting the photos, as you be able to determine the relative age of a photo.
http://www.cycleback.com/pdfbook.html