Double 50% is 100%, so assuming your market doesn't significantly grow, it's impossible to double your market share again.

Well, I wonder if it is true. I mean, theoretically, something could reach 50% drop down to 30% again and double to 60% again. Yes, it is unlikely, but I see no reason why it should not be possible:

http://www.norvig.com/norvigs-law.html

But most certainly that is not how the law was meant to be interpreted.

2x 50% penetration = 100% penetration.