It's getting more and more rare but 5-10 years ago it wasn't. More places are stricly combining their Hem and Onc fellowships, with no option to do one or the other. There are plenty of BC Hem or Onc people out there but most of them trained some time ago. I'm applying now (for Fast Track), in what I'm planning to just be Onc. There are only about 20 programs in ERAS that are listed as Onc only. There are another 20 or so in the Hem/Onc category that offer an Onc only option. I'm sure these places offer similar Hem only options but can't swear to it as I'd rather do an STD fellowship than one in Hem only so I didn't check it out.
FWIW, if you're only interested in academic medicine and research, single boarding is probably just fine. If you have any aspirations to do private practice you should probably dual board. I'm debating the issue right now myself and the more people I talk to, the more convinced I am that 6 months of benign hematology torture would be worth it in the long run. I'm not completely convinced yet but we'll see.