I have on my macbook air 2 iphoto libraries. 1. Iphoto library and 2. Iphoto library (original). The capacity of the iphoto library is twice als much as with command-I. Can I delete the original?

If you've confirmed that the newer library contains all of your photos and is working as it should there's no reason you can't delete the original library. You do the confirmation by launching iPhoto to open the newer library and checking to see if it contains all of your photos and they can be edited and used in slideshows, etc.

Rename the original library to "Original Library" and launch iPhoto with the Option key held down. Select the Original Library in the Open window and check to see what it contains. Also check the trash bin in it.

Do the same for the other library. Then put the Original Library in the Dock's Trash bin but don't delete. Open the other library as noted above and confirm it has all of your photos, you can edit them, etc. If all is well with that library empty the Trash.

But in the finder the library has 100 GB and the original had 0 GB (but when I click on the original (with the 0 GB) it goes to my full library with the 50 GB

It looks like your "original" library were just a link. Does the icon show with a little curved arrow in the lower left corner?

If yes, then select alias and enter the key combination ⌘R, for "File > Show Original". Where does that take you?

Provide us with a screenshot of your two iPhoto Libraries as seen in the Finder.

Open the original library again and then Control (right) - click on the iPhoto title in the top frame to see which library is open. It'll look like this:

This is to make sure you're in the right library.

To take a screenshot of your Pictures folder with the two libraries, open the folder in the Finder. With this folder open as the frontmost Finder window enter the key combination ⇧⌘4

Now select the area around the window to take the screenshot and save it to the Desktop.

Then drag the screenshot into the window of your next answer.

And the other fact is that the library is twice as the volume of the pictures.......

The size you are seeing in iPhoto is much smaller than the size of the actual iPhoto library.

iPhoto is showing an estimate of the size that will be needed, if you export all photos from iPhoto. But the library may be actually twice the size of this estimate. iPhoto is storing all original image files, that will be larger than the edited versions, it is storing additional previews for sharing with other applications, and smaller thumbnails to speed up browsing.

If your main concern is the amount of storage required to store your iPhoto Library, then you cannot do much about it, other than deleting photos that are redundant and you do not need any longer.