fdo#85496: editeng: avoid exporting duplicate attributes

Since commit 0d57434180db6c8eda8c5b9b704f8a1c18b371df multiple 0-length
attributes will be exported by the ODF filter as duplicate attributes.

(cherry picked from commit 7a242b463132d67a4a2d6e69319e0da367145cc0)
(cherry picked from commit 846b56b6b99e334dfa44f1a24640aa3158509854)

This backport takes a different approach from the master fix and simply
detects duplicates in EditTextObjectImpl::GetAllSections() which should
be safe enough for 4.2.8.2.

Change-Id: Iff787c8d2a71bc3082192cc98e3d916badee65dd