Hi
It means that you have 24 ports capable of running at 1Gbps each. You have a 20Gbps connection to the backplane, yes it is full duplex that is how Cisco come to the figure of 720 as in Sup720 eg
6509 =
9 x 40 = 360 ( possible 40Gbps connection to switch fabric per module )
Full duplex = 2 x 360 = 720
So it's not really a 2.4:1 oversubcription it's actually quite close to 1:1. If you left 4 ports free you would have no oversubscription at all.
You could look at the 6748-SFP module but that as 2 x 20Gbps connections at full duplex so it's the same contention rate.
HTH
Jon