I using Foobar, and I have still the same problem ! Memory cache and tracks appears in double. I think we can just wait of the fixing problem by last.fm ... I guess in two or three century...
Edit 17h35 :
I have uninstall scrobbler of my computer et re-install properly it, problem still working xD !
I have read on internet, if the track was put in the memory cache, it's because scrobbler isn't connect to last.fm, do you confirm this ?
18h00 : Apparently, after uninstall and re-install and use only one account on the scrobbler, it's working good, for the moment but still making double. I will check if it's good working the rest of the day, and week end.