Merge r1698334 from trunk:

Avoid adding duplicate subequest filters, as they would not be stripped
properly during an ap_internal_fast_redirect.


Submitted by: covener
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1770672 13f79535-47bb-0310-9956-ffa450edef68