I did some testing to find out what would happen in a couple of different scenarios ... Here's what I found out: What would happen if I try to add a photo in the LR-CC desktop application that I’ve already added to LR-CC on a mobile device? The LR-CC desktop application recognizes the duplicate during import. The duplicate will be greyed out, without a checkmark, and on mouse-over will say “Previously Added”. What would happen if I try to add a photo to LR-Classic that I have already added to LR-CC? If you’ve activated “Sync with Lightroom CC” in LR-Classic, then LR-Classic should identify this photo as a duplicate, and it will be dark and not selected when you attempt to import photos from the card or folder where that image resides. (Note: If the image is renamed, it will not be identified as a duplicate) What would happen if I try to add a photo to LR-CC that I have already added to LR-Classic? If that photo is in a collection in LR-Classic that is synched with LR-CC, then LR-CC will not allow you to add the photo and instead you’ll get a “Skipped importing duplicate” message. (Nice!) However, if that photo is not in a synced collection in LR-Classic, but you’ve activated “Sync with Lightroom CC” in LR-Classic, then the LR-CC photo would become a virtual copy of the existing photo in LR-Classic, but the actual image itself will not be duplicated. I’ll explain this in greater detail below … In LR-Classic, after syncing with LR-CC, the photo would appear in the “Synched Duplicates” collection in the Catalog menu. If you single-click the image, then go to “All Photographs” in the Catalog menu, the duplicates will be highlighted. The photo from LR-CC will become a virtual copy of the photo that was already in the LR-Classic catalog. In this case, the original image will not download to the LR-CC download location on your computer, since you already have the image on your computer. If you delete the synched LR-CC photo in LR-Classic, which is considered a virtual copy in LR-Classic, the photo will be deleted in both LR-Classic and Lightroom-CC. Undo delete will not bring it back in LR-CC but it will bring back the virtual copy in LR-Classic. If you delete the LR-Classic photo, that photo will be deleted, AND the synched LR-CC photo will be deleted from both LR-Classic AND Lightroom-CC! Again, undo will not bring it back in LR-CC. Your best option is to either do nothing if the photo has different edits applied to each version and you want to keep both, or keep only the LR-Classic photo and put that in a collection that is synced with LR-CC. If you prefer the edits of the LR-CC photo, copy/paste the edits from the LR-CC photo to the LR-Classic photo before you delete the LR-CC photo. If I’ve activated “Sync with Lightroom CC” in LR-Classic, where are the files downloaded? The images are downloaded to wherever you specified in LR-Classic Preferences à Lightroom Sync. If you want to move the images to another folder after they’ve downloaded, you can do so in Lightroom Classic, but not outside of Lightroom or else it won’t be able to find the images. Changing the LR-CC downloads location will only impact future downloads. It will not move or impact the images that have been previously downloaded to another location.
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