For Freewind

reorganizelist.scala

case class Message(username:String, content:String)
val list = List(Message("aaa", "111"), Message("aaa","222"), 
   Message("bbb","333"),Message("aaa", "444"))


import collection.mutable.ListBuffer

list.foldLeft(new ListBuffer[(String,ListBuffer[Message])]()){ 
  case (buff, item) => 
    if(buff.nonEmpty && buff.last._1 == item.username) buff.last._2 += item
    else buff += (item.username->ListBuffer(item))
    buff
}.map{pair => (pair._1, pair._2.toList)}.toList

case class Message(username:String, content:String)
val list = List(Message("aaa", "111"), Message("aaa","222"),
Message("bbb","333"),Message("aaa", "444"))

if (list.isEmpty) Nil
else
for (List(m1, m2) <- list.sliding(2) )
yield if (m1.username == m2.username) m2.copy(username = "")
else m2