Don't know how to explain this

Hello. I've made a program that reads a chat log on a website, and would like to fetch only the latest 3 messages that have NOT been read by the program yet. What I have at the moment:

Java Code:

String[] lines = data.split(System.getProperty("line.separator"));
Set<String> current = new HashSet<>();

for (int i = 3; i < 6; i++) {
    if (lines.length < i)
        return current;

    if (current.contains(lines[i]))
        continue;

    current.add(lines[i]);
}

Here's the 3 latest lines. Now before I add the line to the 'current' HashSet, I would like to somehow check that the line was not in the previous 3 lines. Basically I don't want to see duplicated lines.

This method is called whether there are new lines or not, so simply creating a List/Set of the 3 previous messages will not work.

How would I go about doing this?

Edit: came up with a rather simple way.

Java Code:

String[] lines = data.split(System.getProperty("line.separator"));
Set<String> current = new HashSet <>();

for (int i = 3; i < 6; i++) {
    if (lines.length < i)
        return current;

    String line = lines[i];

    if (current.contains(line))
        continue;

    if (latestMessages.contains(lines[i]))
        continue;

    current.add(line);
}

latestMessages.addAll(current);

if (latestMessages.size() >= 7) {
    for (int i = 0; i < latestMessages.size() - 3; i++) {
        latestMessages.remove(i);
    }
}