how can i remove duplicate photos?

There is no built-in tool to detect duplicates in Photos. If you have too many duplicates to find them by sorting the photos by date, you could try to use third party duplicate Finders.

Photos is very new, and there are only a few apps, that have been updated to handle Photos Libraries are:

For example, these new apps claim to work with Photos:

• Power photos: Fat Cat Software
• Duplicate annihilator: Brattoo Propaganda Software - Products
• PhotoSweeper: http://overmacs.com/photosweeper.html

But have not tested any them yet, so no guarantees given.

I personally recommend PhotoSweeper (available on the Mac App Store).

It is super fast, has a great search and selection criteria before running the find duplicates, and you can also mark / lock photos first so you ensure the one that will be deleted will not be the locked one! Furthermore, it doesn't actually delete the photos, but puts them into it's own album "PhotoSweeper Trash", so you can double-check everything before actually deleting yourself in Photos.

I have tried Duplicate Annihilator, but I found it painfully slow, and it is subject to the problem when using "selection" - so it's hard to only do a sub-set of your library.