Don't process content items multiple times in the scheduled job

[rbottema]

A content item can be tagged with multiple tags, which would make them end up multiple times in the GetTaggedContentGuids list. Filtering out duplicates cleans this list and avoids doing duplicate processing. This speeds up the job greatly, especially in sites with heavy tag usage

[rbottema]

There are more potential performance improvements that are worth looking into, but I didn't have time for. Examples are:

• Loading content types only once per content type
• Constructing a lookup of GUID -> tags to avoid multiple looping over all tags