Hi louiseapl
welcome to the Apple (user-to-user) Community.
Re: "tagging individual twins in photo on iphone" :
" ... would you have any ideas how to add a tag when the two people in the picture look very similar?"
Congratulations on your twins!
Sorry I don't know how this will work with twins, however:
This link below opens the newest iOS 17 article on identifying people in iPhone's Photos app:
How to: Identify people and pets in Photos on iPhone - Apple Support
Eg: says:
Name a person or pet in a photo or video
When you name a person or pet in a photo or video, they’re automatically added to your People & Pets album and identified in other photos and videos in your library.
- Open the Photos app
on your iPhone. - Open a photo or video, then tap

- or swipe up to see the details.
- People or pets already named appear in the bottom-left corner of the photo.
- A question mark appears next to those you haven’t named.
- Tap the person or pet with a question mark next to their picture, then tap Tag with Name.
- Enter their name, tap Next, then tap Done. "
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Also: Use the People & Pets album in Photos on your iPhone, iPad, or iPod touch - Apple Support
says:
"Put a name to a face : Photos suggests names from your Contacts,
but you can manually add a name to a person that's already in your People & Pets album:
- Open the People & Pets album, then tap the thumbnail of the person that you want to name.
- Tap Add Name at the top of the screen.
- Type the person's name or select their name if it shows up.
- Tap Next, then tap Done."
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There's a discussion thread: re: Adding notes and tags to photos - Apple Community
Eg: says:
"Photos for iOS on the iPhone cannot display keywords, titles, caption. So it has no tool to add them.
(... does have more info. about using Mac)
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For more info. / support etc., you may want to reach out to the Photos Support staff via their website:
(Eg: online chat / phone / email etc.) : Photos - Official Apple Support
All the best :-)