No repeat nicknames

For real usernames, remove already used nicknames from the list once chosen and update linesSize. To prevent IndexOutOfBoundsException from multi-threading make nextReal method synchronized.

src/main/java/me/creepermaxcz/mcbots/NickGenerator.java

@@ -78,10 +78,13 @@ public class NickGenerator {
         return prefix + generateRandom(nickLen);
     }
 
-    public String nextReal() {
+    public synchronized String nextReal() {
-        String nick = prefix + lines.get(random.nextInt(linesSize));
-        return nick.length() <= 16 ? nick : nick.substring(0, 15);
 
+        int index = random.nextInt(linesSize);
+        String nick = prefix + lines.remove(index); // Remove chosen nicknames
+        linesSize = lines.size();
+
+        return nick.length() <= 16 ? nick : nick.substring(0, 15);
     }
 
     /**