A little more information would be helpful. By "Duplicate option does not appear under Utilities," do you mean that

Hidden, Recently Deleted and Receipts do appear, but Duplicates is not there? That usually means that Photos has not found any images that it thinks are duplicates.
What did you do when you saw that Duplicates were found? Did you Merge them or Delete them or leave them alone? Was the "once with 2 Photos" two of the same ones that you saw before, or new finds?
When you say that you have "a ton of duplicates," do you mean identical, or similar images with somewhat different resolutions or filenames or something? How large is your Library?
How much free storage do you have in your MacAir? Mine says 775 GB of 1 TB used, for instance.
Do you use iCloud Photos? Do you have Optimize Mac Storage turned on for Photos? How much free space is there in iCloud?
Where is your Photos Library stored? Is it in your Pictures folder, or is it on an external drive?
Have you tried making a a new duplicate (⌘-D) picture, and waiting to see when and if that shows up?
I made a duplicate when I started to read your post, and I labeled it Dupe in the Title. It shows up in Recently Saved. I'll see when it appears in Duplicates.
Let us know the extra information…