tricky lookup formula

[twl2009]

I am trying to find a formula, to be dragged down a col N to produce a list, that will do 2 things.

Firstly look in col P5:P for any positive number.

Secondly return the value that is in col C of same row, but only if value hasn't already been returned.

Does this sound possible. (vague illustration below)

C________P____________N


ab10_____1___________ab10
ab10_____2___________ab12
ab11_____0___________
ab11_____0___________
ab12_____1___________
ab12_____0___________

 

[twl2009]

i am sure there is a combination of INDEX, MATCH, SMALL and IF that will make this work, but I just cant make it work