Package thumbnail image is copied twice (and produced warnings) if placed in META-INF/vault/definition folder

scenario:

• content package project with a custom thumbnail image stored in the source at META-INF/vault/definition/thumbnail.png
• the plugin config contains a config parameter

  <thumbnailImage>src/main/content/META-INF/vault/definition/thumbnail.png</thumbnailImage>
  

this produces a warning in the package goal like:

[INFO] Packaging content from C:\temp\mysite\ui.content\src\main\content\jcr_root
[WARNING] Found duplicate file 'META-INF\vault\definition\thumbnail.png' from sources 'C:\temp\mysite\ui.content\src\main\content\META-INF\vault\definition\thumbnail.png' and 'C:\temp\mysite\ui.content\target\vault-work\META-INF\vault\definition\thumbnail.png'.

this does not happen when the thumbnail image is copied from somewhere else, but it is convenient to put it rightaway in the correct folder of the source project.