A library is missing from dependencyClasspath if it's added to libraryDependencies twice in ascending order

For example, if different versions of scalatest are added in ascending order, then it's not included in dependencyClasspath.

````
libraryDependencies ++= Seq(
  "org.scalatest" %% "scalatest" % "2.2.1" % "test",
  "org.scalatest" %% "scalatest" % "2.2.2" % "test"
)
````

````
> show test:dependencyClasspath
[info] Updating {file:/Users/kawachi/tmp/xxxxx/}xxxxx...
[info] Resolving org.fusesource.jansi#jansi;1.4 ...
[info] Done updating.
[info] List(Attributed(/Users/kawachi/tmp/xxxxx/target/scala-2.10/classes), Attributed(/Users/kawachi/.sbt/boot/scala-2.10.4/lib/scala-library.jar), Attributed(/Users/kawachi/.sbt/boot/scala-2.10.4/lib/scala-reflect.jar))
````

When it is added in descending order, the newer is in dependencyClasspath.

Ref. https://groups.google.com/d/msg/sbt-dev/K7ji0gLOvV8/mr8a2Z3jszAJ