Start-Class and Main-Class can end up pointing to same place

[dsyer]

$ cd spring-boot-samples/spring-boot-sample-simple
$ mvn clean install package # note the mistake
$ java -jar target/*.jar
<BANG!>

[philwebb]

Most probably due to the repackage jar task trying to repackage an already repackaged jar. Probably the easiest fix is to bail if 'Main-class' is already set.

Probably also worth checking that nested jars are not added multiple times.