[doxyrest] avoid adding derive compounds multiple times
[doxyrest] avoid adding derive compounds multiple times

doxygen may have multiple <derivedcompoundref> with the same refid (due
to template specialization and all) -- we don't want any of that.

vovkos/doxyrest,vovkos/doxyrest,vovkos/doxyrest,vovkos/doxyrest,vovkos/doxyrest