sbt-aseembly を使うと

[error] (*:assembly) deduplicate: different file contents found in the following:
[error] /Users/ryoco/.ivy2/cache/io.netty/netty-handler/jars/netty-handler-4.0.41.Final.jar:META-INF/io.netty.versions.properties
[error] /Users/ryoco/.ivy2/cache/io.netty/netty-buffer/jars/netty-buffer-4.0.41.Final.jar:META-INF/io.netty.versions.properties
[error] /Users/ryoco/.ivy2/cache/io.netty/netty-common/jars/netty-common-4.0.41.Final.jar:META-INF/io.netty.versions.properties
[error] /Users/ryoco/.ivy2/cache/io.netty/netty-transport/jars/netty-transport-4.0.41.Final.jar:META-INF/io.netty.versions.properties
[error] /Users/ryoco/.ivy2/cache/io.netty/netty-codec/jars/netty-codec-4.0.41.Final.jar:META-INF/io.netty.versions.properties
[error] /Users/ryoco/.ivy2/cache/io.netty/netty-codec-http/jars/netty-codec-http-4.0.41.Final.jar:META-INF/io.netty.versions.properties
[error] /Users/ryoco/.ivy2/cache/io.netty/netty-transport-native-epoll/jars/netty-transport-native-epoll-4.0.41.Final-linux-x86_64.jar:META-INF/io.netty.versions.properties
[error] Total time: 7 s, completed 2016/09/21 15:16:13

などと出て困ってたけど、 assemblyMergeStrategy に

assemblyMergeStrategy in assembly := {
  case "io.netty.versions.properties" => MergeStrategy.concat
  case x =>
    val oldStrategy = (assemblyMergeStrategy in assembly).value
    oldStrategy(x)
}

にしても解決しなかったが、

assemblyMergeStrategy in assembly := {
  case "META-INF/io.netty.versions.properties" => MergeStrategy.concat
  case x =>
    val oldStrategy = (assemblyMergeStrategy in assembly).value
    oldStrategy(x)
}

にしたら解決した。META-INF をつける。