adds a duplicate header on reruns

[viboga]

I have a custom copyright header with following configuration for my projects:

headerLicense := Some(HeaderLicense.Custom(
"""|---------------------------------------------------------------------------------
|...
|---------------------------------------------------------------------------------""".stripMargin))
headerMappings := headerMappings.value + (HeaderFileType.scala -> HeaderCommentStyle.cppStyleLineComment)
headerEmptyLine := false

I have a root project (build.sbt) which doesn't have any scala files. I added the following to avoid #153.

headerLicense := Some(HeaderLicense.Custom(""""""))

All the old files already have the header. I want to use this plugin for new files which seems to be not working. 'headerCreate' adds a duplicate header to all the old files too. It adds a duplicate to every file on rerun as well.