The URI

mongodb://mongos1.example.com,mongos2.example.com/?readPreferenceTags=foo:bar&readPreferencetags=dc:ny

If there are two entries in the URI for readPreferenceTags, and one is case-insensitive it is not appended to the parsed readPreferenceTags

This returns:

{ readPreferenceTags":[{"dc":"ny"}] }

Should return:

{ readPreferenceTags":[{"foo": "bar"}, {"dc":"ny"}] }

 
I believe this occurs because the query parser will gather same-value key as an array. But because they don't have the same key the latter value is overwriting the previous.