Is it possible that a Photos library stored on an external disk will duplicate itself on the computer hard drive if backed up to iCloud?
Tell us more about your external drive and the 130GB USB disk.
Where is your Photos Library stored? Is it on the USB Disk? If you keep the library on an USB disk, Photos may have created a second library on your system drive, because the USB disk is not eligible for an iCloud Library (This is new in this support document since High Sierra. I do not know why)).
See: Move your Photos library to save space on your Mac - Apple Support
Apple doesn't recommend storing photo libraries on external storage devices like SD cards and USB flash drives, or drives that are shared on a network.
You can't store your library on a disk that's used for Time Machine backups.