Hi Csoto,
There are few things to check in this type of situation, including File Name, File Size, Hash Values.
If you right click the patch in patch management and choose download, it should skip it, but once it does that, does the value change to Downloaded - Yes?
I have seen on a couple occasions the file names of patches be very very close, but not quite exactly the same. It might be worth opening the patch content in patch management, copying the name value shown there, and renaming the file to match.
Regarding File Size and Hashes, you can obtain this information by quering the SQL Database directly. I adapted the sql query from this document to return the MD5 (hash), SHA1, SHA256, and FileSize values.
SELECT Vulnerability.Vul_ID, Vulnerability.Guid, patch.UniqueFilename, patch.Hash, patch.SHA1, patch.SHA256, patch.FileSize FROM Vulnerability INNER JOIN Patch ON Vulnerability.Vulnerability_Idn = patch.Vulnerability_Idn WHERE patch.Name LIKE '%6u91%'
You could take a look at the returned values from this query to see if there are any inconsistancies between things such as file size and hash values. Use your favorite hash validation program to compare the hashes (example: http://sourceforge.net/projects/quickhash/).