Will the hard link file consume the same size of origin file? already known that the hard link is just another name for linked file.
But what upset me is that when select both the original file and the hard link file the size finder shows is doubled.
So is it just a illusion or a real case?
Basically the Finder is unaware of any hard links (at least when summing up file sizes), so each file/link just counts on its own. If you happen to have several hard links pointing to the same physical file in your selection, the calculated size will be bigger than the space used on disk.

Let's say you have a folder containing two hard links to the same file data:



If you run "Get Info" on the folder in Finder, you'll get the sum of both file sizes, even though they only use space on disk once:



So yes, Finder doubles the size (or multiplies it in cases with more than two hard links).

While it might seem easy to detect such situations within one folder, it's not so easy in cases where you sum up the size on a top folder of a deep folder hierarchy. Of course it would be possible to keep track of all physical files counted (which all have a unique id), but in practice the use of hard links is rare so it's probably just not worth the hassle. The only places I've found hard links on my Mac are


hard linked image files from migrating from iPhoto to Photos
Xcode documentation files
Terminfo and timezone definitions
Safari caches