TreeSet Issue

Item.scala

object Item {
  implicit val ord = new Ordering[Item] {
    def compare(i1: Item, i2: Item): Int = {
      if (i1.equals(i2)) {
        0
      } else {
        val order = i1.added.compare(i2.added)
        if (order == 0) 1 else order
      }
    }
  }
}
case class Item(group: String, something: String, other: String, added: Long) {
  override def equals(other: scala.Any): Boolean = {
    other match {
      case i2: Item => group == i2.group && something == i2.something && other == i2.other
      case _        => false
    }
  }
  override def toString: String = s"Item($group, $something, $other, $added)"
}

ItemSpec.scala

import java.time.Instant

import org.scalatest.{ MustMatchers, WordSpecLike }
import org.scalatest.mockito.MockitoSugar

class ItemSpec extends WordSpecLike with MustMatchers with MockitoSugar {

  "Item" should {

    "be unique" in {
      val backlog = items("group1", 1)
      println(backlog.mkString("\n"))
      backlog.size mustEqual 1

      println("===================================================")

      val backlog2 = backlog ++ items("group2", 1)
      println(backlog2.mkString("\n"))
      backlog2.size mustEqual 2

      println("===================================================")

      val backlog3 = backlog2 ++ items("group3", 1)
      println(backlog3.mkString("\n"))
      backlog3.size mustEqual 3

      println("===================================================")

      val backlog4 = backlog3 ++ items("group4", 1)
      println(backlog4.mkString("\n"))
      backlog4.size mustEqual 4

      println("===================================================")

      val backlog5 = backlog4 ++ items("group1", 1)
      println(backlog5.mkString("\n"))
      backlog5.size mustEqual 4

      println("===================================================")
    }

  }

  private def items(group: String, num: Int) = TreeSet(Seq.tabulate(num)(i =>
    Item(group, s"something$i", "other", Instant.now().toEpochMilli)
  ): _*)

}

output.txt

Item(group1, something0, other, 1521477246485)
===================================================
Item(group1, something0, other, 1521477246485)
Item(group2, something0, other, 1521477246491)
===================================================
Item(group1, something0, other, 1521477246485)
Item(group2, something0, other, 1521477246491)
Item(group3, something0, other, 1521477246492)
===================================================
Item(group1, something0, other, 1521477246485)
Item(group2, something0, other, 1521477246491)
Item(group3, something0, other, 1521477246492)
Item(group4, something0, other, 1521477246492)
===================================================
Item(group1, something0, other, 1521477246485)
Item(group2, something0, other, 1521477246491)
Item(group3, something0, other, 1521477246492)
Item(group4, something0, other, 1521477246492)
Item(group1, something0, other, 1521477246492)