Importing images from external drive

You can't avoid them being (initially) duplicated. When importing images into Lightroom, from whatever source, the program initially copies the files into it's own designated location (default is the system drive, but this can be changed in Preferences to a different drive). From that point Lightroom has no interest or involvement with the original input files (so you are free to delete them yourself if needed). It's from that designated location that the images are then synced up to the cloud. Once synced, the local copy becomes eligible for removal, though that is done under some complex rules....which means it can take a long while before they are automatically deleted. They can, however, be deleted manually by the user....though of course it's vital to make sure they have safely been uploaded to the cloud before deleting them. 

Personally, I've got my designated location set to an external drive with lots of free space, so I'm quite happy leaving any locally stored originals there. It's also worth mentioning that Lightroom also downloads a copy of the original when you start to edit it, which helps with performance....another reason why I don't mess with the local contents of my designated folder .