I actually dug up a Comet CF-4160J Diplexer (both Comet and Diamond call them "Duplexers") and swept its response with my Spectrum Analyzer, using the Low-Frequency and Common Port. I was also curious to find out if it mattered if the High-Frequency Port was terminated or not.
The bottom line is that this particular Diplexer would reduce the 2-meter 3rd harmonic by 55 dB, and had an insertion loss of 0.12 dB. And I confirmed that there was a small reduction performance if the High-Frequency Port was not terminated.
I would expect the performance of Diamond models would be similar and I will test one when I find one.
The actual report can be found here: http://wb4son.com/wpblog/?p=2828 (if the link is blocked, try WB4SON dot com and search for CF4160)
73, Bob, WB4SON