Hello Ewan Lamont,
We understand you need help with removing duplicate pictures on your Mac system. Here is the guide with steps specific to Mac: Remove duplicate photos in Photos on Mac
1. In the Photos app on your Mac, click Duplicates in the sidebar.
Note: If you’re using iCloud Shared Photo Library, click the Library pop-up menu in the toolbar, then choose Both Libraries.
2. Select the duplicates you want to merge.
You can select individual items in a single row and merge them or select multiple rows of duplicates and merge them all at once. A single original photo or video is kept from the selection in a row and other duplicates are deleted.
3. Click Merge [number] Duplicates.
One original photo or video appears where the selected duplicates were located in your library. Deleted duplicates appear in the Recently Deleted album where you can recover or permanently delete them.
If you continue to have issues, you may need to work directly with Apple Support. You can reach them here: Get Support
You can also find the support phone number for your region at this link: Contact Apple for support and service
All the best.