Hi there, lky_12april,
Thank you for stopping by the Apple Support Communities. I understand that while managing your picture library, you've gotten duplicate photos on your iPhone. I'm happy to help!
It sounds like you are following the right steps to import the pictures to the computer from the phone, clear the phone of the photos, then sync back only selected photos from the Mac. While there are a number of reasons this could be happening, there are two that pop up immediately for me. Those are:
- The pictures are duplicated in the Photos app on your Mac, or
- The pictures are not deleting as expected from the iPhone
Given that, the first thing I recommend is checking to see if the album(s) being synced from the computer have inadvertently obtained duplicate photos. If so, remove the duplicates from the computer: Use Photos on your Mac - Apple Support, then sync your phone again: Sync your iPhone, iPad, or iPod using your computer - Apple Support.
If the duplicates appear on the phone, but not on the Mac, I'd suggest turning the sync settings off for Photos. Just connect your phone and follow the steps to set up syncing from Sync your iPhone, iPad, or iPod using your computer - Apple Support, but in reverse. You will want to confirm/apply the sync. This will remove pictures synced from the computer and allow you to verify if the duplicates were just never deleted from the phone. If so, delete the pictures on the phone: Delete photos on your iPhone, iPad, and iPod touch - Apple Support, then sync again.
If you still are seeing duplicate pictures even though they aren't any duplicates in Photos on your Mac, and/or the phone itself doesn't have any pictures stored, I'd next see if you're able to successfully sync (with no duplicates) from safe mode: How to use safe mode on your Mac.
Don't hesitate to respond back if you still need some help.
Cheers!