Can't you have duplicate words with different meanings in different
expansion sets?  Is this a bug?

Please try this repro below.  It uses a thesaurus with 2 expansion sets.
Each set contains the word kind.  The sets are like this:
1. kind, sort, class
2. kind, caring, considerate

When I set up a thesaurus with the above two sets (and resart the FTS
service) I only get one row from this query:

SELECT *
FROM fts_bug
WHERE CONTAINS (txt, 'FORMSOF(THESAURUS , "kind")');

I would have expected to get five rows (each row has a single word): kind,
sort, class, caring, considerate

------------------------- THESAURUS REPRO ----------------------------------

CREATE TABLE [dbo].[fts_bug](
[id] [int] IDENTITY(1,1) NOT NULL,
[txt] [varchar](50) NULL,
CONSTRAINT [PK_fts_bug] PRIMARY KEY CLUSTERED ([id] ASC)
)

-- catalog and index
create fulltext catalog testCat as default;
create fulltext index on dbo.fts_bug(txt) key index PK_fts_bug;

-- populate
insert into fts_bug(txt) values ('kind')

insert into fts_bug(txt) values ('sort')
insert into fts_bug(txt) values ('class')

insert into fts_bug(txt) values ('caring')
insert into fts_bug(txt) values ('considerate')

-- see data
select * from fts_bug

-- Use this thesaurus (restart the FTS service!):
<XML ID="Microsoft Search Thesaurus">
   <thesaurus xmlns="x-schema:tsSchema.xml">
<diacritics_sensitive>0</diacritics_sensitive>
 <expansion>
  _kind
  _sort
  _class
 </expansion>
 <expansion>
  _kind
  _caring
  _considerate
 </expansion>
</thesaurus>
</XML>

-- I would EXPECT this query to return "kind", "sort", "class", "caring" and
"considerate"
-- but it only returns one row "kind"
SELECT *
FROM fts_bug
WHERE CONTAINS (txt, 'FORMSOF(THESAURUS , "kind")');

-- Now use this one: (I only changed the first "kind" to "kinds") (restart
FTS service)
<XML ID="Microsoft Search Thesaurus">
<thesaurus xmlns="x-schema:tsSchema.xml">
 <diacritics_sensitive>0</diacritics_sensitive>
 <expansion>
  _kinds
  _sort
  _class
 </expansion>
 <expansion>
  _kind
  _caring
  _considerate
 </expansion>
</thesaurus>
</XML>

-- Running the same query now returns 3 rows: "kind", "caring",
"considerate", which you would expect
SELECT *
FROM fts_bug
WHERE CONTAINS (txt, 'FORMSOF(THESAURUS , "kind")');

-- clean up
drop fulltext index on dbo.fts_bug;
drop fulltext catalog testCat;
delete  from fts_bug
drop table fts_bug

------------------------- END THESAURUS
REPRO ----------------------------------