#1
Say you have a lick like this:
E||-----------10-14p10----------||
B||--------12----------12-------||
G||-----11----------------11----||
D||--12--------------------------||
A||-------------------------------||
E||-------------------------------||

I have trouble on the D string when I am changing directions because I find it more natural to hit the string twice as I do on the e string in this pattern. I am wondering if it is "wrong" to play the D on the D string twice. Thanks!
#2
Well, it's "wrong" if you have to play the D string twice for a cover (wrong if you're planning to cover the original exactly, obviously), and if it's your own music you can do whatever you want. Simple.

Of course, you should be able to do it both ways.