Can I erase old library?
Is the system duplicating the old library, can I erase it (to save HDD space)?
Thanks
Photos will create a new library, but the old iPhoto Library and the Photos Library are stired very efficiently using hard links. The original Master files and previews are stored in the same blocks on the drive, so the storage will not increase much.
You can delete the iPhoto Library, but it will not free much storage as long as teh same photos are also in Photos. I'd wait, until you have checked if the migration went well and keep a backup of the original library.