Can Dragonslayers eat their own magic? Early in the anime, Lucy asked Natsu if he could eat his own fire.  Natsu explained that it would be like Lucy eating one of her Celestial Spirits. While this tells us that Natsu won't do it, it leaves the possibility open.

Given how Angel treated her spirits, I could imagine a more evil wizard being willing to eat their spirit if desperate enough.  So, could a more evil dragon slayer eat their own magic?  Would it make them more powerful or just back to the state they were in prior to casting the spell?
I think they can, but they just wouldn't gain any energy like they would from other sources, so it's just completely pointless. There have been points with dragonslayers in pretty tight pinches, and I'm pretty sure one of them would have at some point eaten their own magic if there were any point in doing so/ if it's even possible. 

I think Natsu was just making a bit of a stupid analogy when he said it was like Lucy eating celestial spirits, at no point has there been any suggestion that a dragonslayer's element is in some way alive, and he's more than happy to fire off bursts of fire that presumably must burn out eventually, I don't think ethics come into it at all, just practicality.