Duplicates vs Duplicate Marked Photos

As shown, "Duplicate Marked Photos" is just a normal Album that you (or maybe some app you ran) create at some time in the past few months.

You are correct, the "Duplicates" special view does not appear if Photos has not yet detected any duplicate photos or videos (or you have dealt with all that it has detected).

Can you tell us more about what you mean? Where do you see these phrases? I don't have them, and I haven't seen them mentioned, before. You are using the Photos app on a Mac, is that right? Photos does not "mark" any pictures as duplicates. What OS are you running? We sometimes make duplicates so that we can try a different crop or edit, and we may include "Dupe" or something in the caption or title. But this is entirely for personal use. Perhaps you're seeing pictures that someone else as labeled.

Has it been a couple of weeks since you've installed your OS? It takes a week or more before duplicates are identified. If it's been a while and you don't have a Duplicates folder, then Photos hasn't found any exact duplicates. Pictures may appear alike to us, but they might have different resolutions or sizes or crops or edits that keep them from being identified as real duplicates. Photos tries to get past some of these things, and it may identify these as similars and include them in the Duplicates folder, and sometimes the similars aren't what we would call duplicates. Photos does not "mark" any pictures as duplicates.