Lightroom: Collections transferring from other libraries into synched photos

Sounds like you are using Lightroom Classic, and you mean you started with a new ‘catalog’. You can only sync one catalog, and if you break the sync of a catalog by starting to sync another one, the online synced images will not be removed. So next time you do this you must first remove all the images from the ‘All Synced Photographs’ collection in Lightroom Classic, before you start syncing a different catalog.

For the current images that path cannot be used anymore, so the only option is to remove them by using Lightroom CC or going online and doing it via the web.

Sorry if I sound a little stunned, but how do I use Lightroom cc to do this? Or where do I find my photos on the web to do this?

Go back to the old catalog on LR Classic, un-sync the collection first then delete the photos from ‘All Synced Photographs’ then do a sync to tell cloud the pics are removed... all should be cleared from cloud. Switch to the new catalog and do you sync procedure.

If you dont do it that way then when you go back to the old catalog or do another sync then lightroom will add all your old sync photos to the latest sync catalog resulting in duplicate photos in different catalogs... LRcc is a childs toy simply not ready for the serious photographer. You will encounter other anomalies as time goes by and wonder why the process dont seem to make sense.