"I have two of the same photo."
Translation:J'ai une photo en double.
I think using "of" in this sentence is just an English thing, but possibly "J'ai les deux mêmes photos" should work. (Or "J'ai des deux mêmes photos"?)
A closer English translation of "J'ai une photo en double" would be "I have a duplicate photo."
surely there are lots of other ways to write this in French? Also why is it une instead of la since two of the same photo surely refers to a specific photo.
Sure "two of the same photo" refers to a specific photo, but that does not mean it is grammatically definite. Consider "I have a chair." Yes, it may refer to a specific chair, but that doesn't mean one uses a definite article. The "the" isn't there because of "photo"; it's there because of "same." "I have two of the same photo" and "I have a photo in duplicate" are usually indicating the same thing.
The exception to that that comes to mind is when "I have two of the same photo" could also have been phrased "I have two of that same photo," making it more clear that it's actually a third photo (probably in the possession of someone else) to which two additional photos are identical.
Sorry if my explanation is a bit confusing; I'm having a bit of trouble finding the right words.