Running shade twice without clean leads to warnings and errors

Frequently, when I run a build without 'clean', I get a torrent of unwanted 'duplicate symbol' errors from shade. More recently, when I did the same for a build that includes some shading that combines files from META-INF, this turned into a fatal error.

I've managed to produce a repro with maven 3.0.4. Just run 'mvn install' twice in a row to reproduce this. The impression it gives is that, somehow, the shaded jar from the previous build is used as the input, instead of a new jar built from scratch this time around.