Duplicate photos in Photo Library

Yes and no. Yes, in the sense that if you notice duplicates and delete all but one copy, you have to be careful not to delete a copy that you wanted to have in an album. No, in the sense that deleting such a copy will leave no "big black box" or any other indication that there ever was a copy of that photo in that album.

If you use the duplicate handling that was introduced with Photos 8 in Ventura, it is supposed to handle this issue, making sure that the survivor of a set of duplicates remains in any album that held any member of the set.

You will also have to be careful, when you delete photos that have been used in projects like slideshows. Just like albums, the slideshows are just referencing the photos used in the projects, and I am not sure, if even the built-in merging of duplicates will take care of photos in projects. There will not be black boxes in your albums, when you accidentally remove a photo from an album, the album will just have less items and you may not even notice that a photo is missing, but in a book you may be seeing empty place holders, depending on the app you are using to create the project.

As there is no tool in Photos to test, if a photo has been used in a project, I would always create projects in a separate Photos Library with just the photos used for the project, to keep the photos safe.