[O] subtree-export limitations

[Philipp Kroos]

Hi,

currently the support for subtree export is somewhat limited due to the
fact that individual EXPORT_* options are allowed only once.
I.e., in the following the second latex-header-property will be ignored:

* Some subtree
  :PROPERTIES:
  :LATEX_CLASS: scrartcl
  :EXPORT_OPTIONS: toc:nil
  :EXPORT_LATEX_HEADER: \usepackage[ngerman]{babel}
  :EXPORT_LATEX_HEADER: \usepackage{xcolor}
  :END:

So would be any other EXPORT_OPTIONS-line. The responsible function is
org-export--get-subtree-options, which builds a list of already seen
keywords. The lists members are then ignored if seen again.
Is there any particular reason why this is done?

Sorry if this is documented or discussed somewhere already, I couldn't
find any reference that explains this behaviour.

Best regards,
Philipp