How to select rows from a table while ignoring the duplicate field values?

Here is an example:

id          user_id          message

1           Adam             "Adam is here."
2           Peter            "Hi there this is Peter."
3           Peter            "I am getting sick."
4           Josh             "Oh, snap. I'm on a boat!"
5           Tom              "This show is great."
6           Laura            "Textmate rocks."

What i want to achive is to select the recently active users from my db. Let's say i want to select the 5 recently active users. The problem is, that the following script selects Peter twice.

mysql_query("SELECT * FROM messages ORDER BY id DESC LIMIT 5 ");

What i want is to skip the row when it gets again to Peter, and select the next result, in our case Adam. So i don't want to show my visitors that the recently active users were Laura, Tom, Josh, Peter, and Peter again. That does not make any sense, instead i want to show them this way: Laura, Tom, Josh, Peter, (skipping Peter) and Adam.

Is there an SQL command i can use for this problem?