If I edit a photo using Apple Photos, does it take double space on my hard disk (since it keeps the original)?

No. The changes are saved in the database for the library and applied to the preview version. This makes for nondestructive editing, i.e. can revert to the original version easily. When exported the changes are added to the original file to create the edited version outside of the Photos Library.

no

Photos, like iPhoto before it, uses non destructive ediiting and NEVER changes the original - it is always kept untouched

Edits are recorded and stored in the database and applied when you export or display the photo

Same for adding Photos to albums, a photo can be placed in as many albums as you want for organizing and it takes no additional space

Same for duplicating a photo - that simply starts a new edit chain

LN

When editing a photo with Photos, the original is kept. Does it mean that the photo is stored twice on my hard disk? Is it taking double space (or similar)?

The edits you are doing from the adjustment panel in Photos will require only the additional storage for the new thumbnails or Previews.

But you will need twice the storage , if you are using photo editing extensions or an external editor called from "Image > Edit with -- " in Photos. The reason is, that Photos cannot recreate the edits done by the external editor or the extension, so Photos have to store the phot returned by the external editor in addition to the original it sent to the external editor. Be frugal with editing in external editors, if you want to save storage.

Luckily, the adjustments in Photos are so versatile, that I do not have to fall back on external editors too often, so it is not a big problem, that Photos will have to store the external edits as an additional file.

you are welcome

LN