I am a big fan of Beyond Compare. Use it for lots of different scenarios. It is available on Mac and Windows. Use the Folder Compare Option. The interface is fairly intuitive, but time spent getting to understand its various features will be rewarded.
If using the synch or copy features to copy images from one folder to another then you can subsequently use the Lr Synchronise Folder option to import the images. However, if you move or delete files outside of Lr, using Beyond Compare, then the usual painful experience of dealing with the mess created will follow (ie use the copy rather than move or delete features). Consider your own approach as to how you wish to deal with duplicate images in your catalog.
Thanks all!!One way to do that is to temporarily mark each folders photos with a separate label colour. Then create a collection and add all the photos of both folders into it. So you will see together there all the photos from both folders, easily distinguishable by label colour. You can sort these photos on the grid by capture date or filename etc if that helps to group and compare them. Or use the Library filter to view only the photos of a certain file type or camera model.
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[ ...] PhilBurton - Beyond Compare sounds like a great tool outside the lightroom world. I'll keep that on my list of useful utilities. [ ...]