As long as the photo/video is exactly the same, it will not be duplicated in Google Photos. 

When is a photo/video not exactly the same? Let's say you have a photo or video stored in Google Photos and you then created a separate edited version of it (e.g., cropped, resized, filter applied, et al). If you copied the edited photo/video to Google Photos, it would NOT be de-duplicated and would be added alongside your original photo/video.

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