Switch from Photo Stream to iCloud

1 - not if they are truly duplicates - iCloud Photo Library does a good job of catching exact duplicates

2 - yes if they are in a managed Photos library and are supported formats

File types that you can use with iCloud Photo Library

Your photos and videos are stored in iCloud exactly as you took them. All of your images are held in their original formats at full resolution — HEIF, JPEG, RAW, PNG, GIF, TIFF, HEVC, and MP4, as well as special formats you capture with your iPhone, like slo-mo, time-lapse, 4K videos, and Live Photos.

iCloud Photo Library - Apple Support

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Did you edit your photos on the iPhone and your mac? If yes, you may be seeing duplicate edited versions of the photos after you enable licloud photo library on your Mac. But the original image files will not be duplicated in iCloud.

My Photo Stream and iCloud Photo Library are syncing differently.

• Photo stream is transferring only the original and it will not update the photo in the stream, if you edit it later.
• icloud Photo library is syncing all edits and updating the synced version, so these edits need to be synced, when you enable iCloud Photo Library.

Are you taking Live Photos with your iPhone? They will not sync well, if the Live Photos have been transferred already by my Photo stream. For these Photos the video fork will be missing. If you have Live Photos on your iPhone, that gave been transferred to the Mac by photo stream as still frames, I would remove the still frames from the Mac library, before you enable iCloud Photo Library. Or the Live Photos will not be transfered as live.