I made a new master catalogue and imported all my catalogues into it. There are now many duplicates of the same photo - how do I delete all those duplicates?
You must select the duplicate manually. There's no function or filter or seach function to find duplicates automatically.
Axel is correct, this is a manual operation. However, one thing to speed things up (a bit) is in the Library mode, press the right arrow key to advance to the next image, and if you want to delete it, press the "x" key to "reject" the photo. Continue until you finish that folder (or whatever quantum bit you want to work with).
Then Enable Filters (Command/Control-L) and click on the black flag in the Filter region. This will display all of the rejected images. Then press Command/Control-a to make all rejected photos selected and finallly Command/Control-(backspace) to Delete all of the images from your system.
Yes it can be tedious but some nice music and falling into a zen mental mode will make this tolerable.
Adding to above replies-
Be also aware that a Catalog merge has the ability to create Virtual Copies if one single image file was referenced in multiple catalogs with differing Development. So you may see more than one Preview of one image in the merged master Catalog. (ie. not physical files)
The manual 'tidying' of duplicates can be assisted by plugins that can keyword 'Duplicates' and so make them easier to find.
Sorry to disagree with you, Jossbutler, but Duplicate photos fixer will NOT find Virtual Copies in Lightroom-Classic.
Reading a review and TBMK it can only find the duplicates of original physical files in folders.
Running any 'Duplicate Finder' that is not a Lr Plugin can damage your Lr Catalog, and not advisable!
("damage" = Photos showing as 'missing' in the catalog. Wrong files deleted causing lost editing.)
For Catalog safety run ONLY a Plugin Duplicate Finder.
Another possibility is to use a Duplicate Finder program outside of Lightroom Classic to delete the alleged duplicate files from your hard disk(s). Then, in Lightroom Classic the duplicates now appear as a missing photo with the exclamation point icon. Then you can get rid of them in basically a quick sequence of actions: