【Bug?】"An item with the same key has already been added" when applying UNION ALL inside the CROSS APPLY clause

[hiroyuki0415]

SELECT b.businessunitid,
       child.guid
FROM   businessunit AS b CROSS APPLY (SELECT t.teamid AS guid
                                      FROM   team AS t
                                      WHERE  t.businessunitid = b.businessunitid
                                      UNION ALL
                                      SELECT s.systemuserid AS guid
                                      FROM   systemuser AS s
                                      WHERE  s.businessunitid = b.businessunitid) AS child;

When I run this SQL I get the following error:

An item with the same key has already been added

Thanks in advance for your support.