See Old Toad's user Tip: Photos 2.0 vs iPhoto 9.6.1: Features and Capabilities
Photos and iPhoto are very different applications
Apple stopped the development of iPhoto. It is no longer sold and updated. Apple is now developing and supporting Photos as the successor. New Macs come only with Photos. Currently iPhoto 9.6.1 is still working on El Capitan, but any new system update may break it. At the end of March you will no longer be able to order print products from Apple using iPhoto.
While iPhoto is still working, keep it installed, if you are still having iPhoto Libraries. You may need it, if you have problems to migrate a library to Photos and have to fix the problem running iPhoto.
am I just duplicating all my photos ?
If you migrate an iPhoto Library to Photos, the libraries will be sharing the photos by hard links not not needing much additional storage. (Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support)
But in the long run be prepared to switch to Photos for Mac. You cannot rely on iPhoto being available in future system versions. Start using Photos for Mac now, while you are still having both options and the migration is easy.