Improve rem() to avoid duplicate values

Taking the initial example of `@include rem(padding, 0);`, which a developer might want to do if as an easy way to switch between zero and non-zero values, before this patch is applied, the output would be `padding:0;padding:0;`. 

This also works with non-numerical values too - the absurd example of `@include rem(border-style, solid);` would no longer output twice either.