Did Photos duplicate my iPhoto library?

According to that, if I now delete the iPhoto library, for sure I clean at least the space of all those images taken up to 2015 that I afterwords deleted from Photo (i.e. 10 GB). Am I mistaken?

Yes. If you delete the iPhoto Library, the storage will be freed for all photos and videos, that you already deleted from the Photos Library. For your newer photos and videos that still have the hard linked copies in Photos, the storage will not be freed, because the copies in Photos are still stored there.

You should be using Photos - it is better in pretty much every way than iPhoto and iPhoto is a dead product that will totally quit working some day - Photos is a new and different program and has a learning curve and requires different workflows

And to use iPHoto you would have to have version 9.6.1 which since you were not up to date probably is not possible to install

And no, nothing was duplicated - Photos and iPhoto use hard links to share access to the same originals and the same previews - Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

You can delete either library with no effect on the other but it is not recommended until you have used Photos for a while - three or more months

LN

I still have not clear the issue in my mind. I understand that there is an hard link, but what happens when you delete an image on Photos Library? I try to specify it more. After migrating from iPhoto to Photo (i.e. in 2015), I continued uploading photos and videos on Photo. Then I also deleted many videos from Photo (let's say 10 GB). Now, if I access the iPhoto library (it took half a day, since it re-creates the environment), I find photos and videos up to 2015, also videos I deleted on Photo. So, iPhoto library seems more or less a static picture of the library taken on 2015. According to that, if I now delete the iPhoto library, for sure I clean at least the space of all those images taken up to 2015 that I afterwords deleted from Photo (i.e. 10 GB). Am I mistaken?

After you migrate your Photos into your library, you should have the "OK" to delete the application (iPhoto) that you decided not to use. However, backing up images to an external drive is always a good idea. More copies mean more security if you can spare the drive space.

AS I said

And to use iPHoto you would have to have version 9.6.1 which since you were not up to date probably is not possible to install

And as to the finder size that is explained in the link I provided - did you read it?

When Finder reports the file size of your Photos library, it includes all your originals and previews. It may look like your remaining iPhoto or Aperture library is taking up twice the space on your hard drive, but it isn't—your images exist only in one location, even though you may have more than one photo library.

And yes you can delete the old iPhoto library but there is no reason to and it is not recommended until you have used Photos for a while

LN

.can you (or anyone) help me understand why Finder shows the roughly 250GB with both libraries?

Photos creates a new library. Both the old iPhoto Library and the new Photos library contain all photos and can be used independently. But the new image files in the Photos Library are hard links to the image files in the iPhoto Library. A hard link is a full file, the same size as the original, but it is using the same disk blocks as the original file. This way both libraries are having the same size in the Finder, but are sharing the storage efficiently.