I just wanted to chime in that the issue still exists (even though in your link to the bug they say that it was fixed in 1.2).
I was having the same problem. My netflow receiver (PRTG) was consistently showing doubled values. I am running:
pfSense 2.3.2
softflow 1.2.1_2
After editing the softflowd.xml like you suggest and restarting the service, it immediately fixed the bug. My traffic graph no longer shows doubled values.
It is worth noting that I have softflow monitoring two interfaces: the physical WAN and the VPN. The VPN was reporting correct values. Only the physical WAN was showing doubled numbers. But the issue was affecting all three of my pfSense devices.