I also had this kind of problem. In my case it was the cmf_uid index missing in the portal_catalog: so every search was returning all the catalog brains.
So I guess that the problem in the question was related to the fact that multiple objects have been indexed with the same cmf_uid (which is an attribute on the object).
[b.getObject() for b in portal_catalog(cmf_uid='1642')]
should point out the relevant objects.
You should take the objects and verify they have a proper cmf_uid, taking care of possible acquisition problems.