nitdoc: Do not overwrite search results.

Fix the main problem of #882 by avoiding the overwriting of a search
entry when 2 entities has the same name.

TODO: Make the presentation of duplicates less ambiguous.

Note: This PR concerns only the last commit. Others come from PR #907.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>