I am unclear as to what the duplicate check methods are as shown in MythWeb, my assumptions are shown below but I could not find any definitive documentation except for the Only New Episodes.

Check for Duplicates
Option: All Recordings (Help needed here.)
Record if
a. Same as "Previous Recordings" Option???

Option: Current Recordings (I think this is correct.)
Record if
a. It is not currently an available recording.

Option: Previous Recordings (I think this is correct.)
Record if
a. It is not currently an available recording. (AND)
b. It was never in the past recorded.

Option: Only New Episodes (I think this is correct.)
Record only if
a. It has an original air date of less than or equal to 14 days (AND)
b. It is currently not an available recording (AND)
c. It was never recorded in the past.

"All Recordings" is just what it says. It will record duplicates.

The field these choices are in is "Check for duplicates in:" When I select "All Recordings" it does not record duplicates on my machine. I used Sesame Street as a test case because it is on all the time and the doubles do not get recorded. Do you have a different experience?