Photos is creating a new file for the shared album, yes. But the storage for Shared albums is not counting agains your paid Cloud storage subscription. Shared album s are using a separate iCloud Service, that is free.
But the copies of your photos and videos are not necessarily of the same quality as the originals in your Phots Library. Photos is optimizing them for sharing. Large image files will be scaled down to 2048 pixel at the longest edge, and many metadata will be stripped, for example the locations. Shared albums are not suitable to archive your photos. See the paragraph about filetypes you can use in this document: How to use Shared Albums in Photos on your iPhone, iPad and Mac – Apple Support (UK)
"Shared Albums upload a copy of your data. Downloaded content may not contain the same information as the original."