org.eclipse.tycho.compiler.AbstractOsgiCompilerMojo.getClasspathElements() returns duplicate entries

[laeubi]

WHile looking into a JDT issue I noticed that we are passing man duplicate entries in

org.eclipse.tycho.compiler.AbstractOsgiCompilerMojo.getClasspathElements()

more specifically one per target environment. We should really filter this here to not make the live harder, this also blow up the debug outputs. Beside that, it also seems we are passing non exiting directories what JDT complains about.