Sameness relates to unique because it means ‘lack of variety’ which can be classified as different. In many cases, unique is used to describe one that stands out, they are said to be ‘one of a kind’.
Explanation: We are all unique. We all have something different. We all have something individual to offer. Even if other people have the same qualities, simply by the virtue that no two of us live the exact same life (even conjoinied twins) we all have something a little bit different to give.
The audience he was addressing to, was the union and basically the world. It was an oath he had to take as being the 16th president in the United States.
