Executing this request:

http://musicbrainz.org/ws/2/release/4ceb8cb4-98f4-34b5-bec6-992786e1a413?inc=release-groups+media+recordings+artist-credits+artists+aliases+labels+isrcs+collections+artist-rels+release-rels+url-rels+recording-rels+work-rels+recording-level-rels+work-level-rels+tags

Result contains:

<tag-list>
...
<tag count="7"><name>progressive rock</name></tag>
<tag count="7"><name>progressive rock</name></tag>
<tag count="4"><name>psychedelic rock</name></tag>
<tag count="4"><name>psychedelic rock</name></tag>
...
</tag-list>

Each tag is repeated twice in the XML returned document.