Let's say I have pairs (in list) like this: [{a, 15}, {b, null}, {a, null}]

Each pair has a character key and a nullable value. I want to select all pairs where the list does not contain another pair with the same key and a non-null value.

In my sample list above I want to get {b, null} only, because 'a' has a non-null value in the first pair. How would I do that using LINQ?