Can I keep movies on my iPad and not on my hard disk?
For me, the solution for avoiding duplicate storage is to keep all media files (movies, music, pictures) on a "networked storage" device, and then pointing the software app to that location. 

To use iTunes as an example: 

iTunes, Preferences..., Advanced tab 

Click the Change... button next to the iTunes Media folder location 

Use the "pop-up" Finder dialog to navigate to the folder on your network drive where your media files are located. Eventually, you will click the OK button to dismiss the dialog, and set iTunes on its slow grind to hoover up the contents of the media folder you've just specified. However, before you do that, please read the notes below; esp. the second one. 

A couple of notes to conclude: 

Note 1: iTunes is a willful child in that it doesn't "respect" your choice of a designated media folder location. I find that after some time has passed, or I've returned from travel, or... after other apparently random, unspecified and unknown events have occurred, iTunes will revert to its "default" media folder location. I gave up trying to understand this behavior years ago, but it still does behave this way. The only solution I know of is to... change the media folder location again - ad nauseum.

Note 2: There are several "other options" available in the Advanced dialog. In my experience, some of these options are unclear, and have proven counter-productive to my objectives. Rather than rant on this subject, I'll just provide the following screenshot that shows how I've chosen these options. Please also note that YMMV - these options may be perfectly clear to you, and work just as you wish; I'm only saying they don't for me.