Hi there, elsiefromdocklands!
Thanks for reaching out to the Apple Support Communities! That is a great question you have about deleting photos on your device that are duplicate and I am happy to help! I recommend taking a look at these articles for steps to delete photos on iOS devices and on your Mac:
Use Photos on your Mac
Delete photos on your iPhone, iPad, and iPod touch
If this is not the path you are needing to go, feel free to circle back with more information about what’s happening, including if you are using iCloud and if you are seeing the duplicates across all your devices.
Jave a wonderful day!
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iPhone/iPad/ Mac,