Both sentences can evoke a different meaning or tone.
"you have already subscribed to our newsletter."
In this context, the subject (you) is clearly marked by the main verb (subscribe).
It gives the sentence a much more personal meaning.
"I have taken out the trash. Me, no one else but me."
"You have already selected the first option."
Great! It clearly means you. "YOU"!
Notice however that both constructions cannot always work with the same verb.
"You are already done for the night" (correct)
"You have already done for the night" (wrong)
The second one is wrong and it's missing a main verb. In this sentence, the word "have" is an auxiliary verb (linking verb) which usually works best followed by a verb.
"you are already subscribed to our newsletter."
Denotes a less personal tone and opens up another possible interpretation.
That is, someone else could have signed up for a subscription on your behalf.
Perhaps someone did you the favor in signing up for the newsletter in which case "you are already subscribed to our newsletter" would make more sense.