iphoto vs photo avoid duplicate

You can delete the iphoto library after migrating to Photos

Have you added any pics to iPhoto since you started using Photos?

The Photos library is usually a migration of the iPhoto one. Unless you've done something unusual there's nothing to do.

If the migration was performed correctly they are in the Photos library, unless you added any to iphoto after the migration.

1 - the iPhoto and Photos libraries are separate and do not have duplicates - see this article Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

2 - you can delete either without affecting the other

when you migrate both have the same photos (no duplicates but both using hard links to the same ones) - as you make changes to eithr those thanges are not made to the other so once you have made any changes they will be different

THere is no need to delete anything for a while but if you want to you can - it is not recommended

LN

I have also realised that some pictures that I had hidden are not in Photo... Is that normal? Or would them be in other place?

Photos that have been hidden in iPhoto will also be hidden in Photos.

Hidden photos are invisible in the Photos view in Photos, but are visible in the "All Photos" album and any other album, that contains the photos.

If your library contains hidden photos, the numbers of photos in Photos and "All Photos" should differ. Look into the "Hidden Photos" album. You enable the "Hidden Photos" album from the View menu.