I was using Disk Inventory X to visualize my hard drive storage and help me find some extra space.
It appears that iPhoto Library takes 52.0 GB and Photos Library takes 49.4 GB.
Is this a known issue that moving from iPhoto to Photos duplicated everything or is this just a sym link under the covers?
Best Answer
Short answer, no. The measuring tool you are using doesn't take the time to deduct the space saved when files are hard linked, so it over-counts the space used.
The Libraries are using 'hard links' to the real location of the files, so both appear to be approximately the same size, but each actual photo is only located in one place on your hard drive, with pointers from both Libraries.
Deleting either Library will not affect the storage space, only deleting both would actually remove the 50GB of data from your drive.
See: Photos saves disk space by sharing images with your iPhoto or Aperture libraries
More on hard links from Ars Technica: