Chris is correct.
1 means, you are telling someone that right now, they know what they knew before. They know right now, because "know" is present tense. But, they also knew before, because of "already."
2 means you are telling someone that they have known since a point of time in the past.
The first sentence is emphasizing the present moment.
The second sentence is emphasizing the past moment.
Both sentences are talking about knowledge that started at a point in the past and continued up to the present moment.