I tried again today and still get rtl1 is already in use.
As I understand it, the problem is usually caused by using Zadig to install the WinUSB driver for both the SDR's "radio" and for its IR remote control.
When running Zadig, under Options, make sure to select "List All Devices"
and unselect "Ignore Hubs or Composite Parents".
Then choose the Composite Parent (for the SDR device) and install the driver.
Additionally, when the "already in use" error is seen (and is due to installing the WinUSB driver for the IR control), the SDR can be accessed by bumping the device number up by one, so if
FMP24 -i1 is giving that error, use
FMP24 -i2 instead. You can also use
FMP24 -i0 (use first available SDR)
FMP24 -h
...
Options:
-i0 Use first available RTL SDR device [-i0]
-i<num> Use RTL SDR device specified by <num> (1-255)
-i"<ser>" Use RTL SDR device with specified serial number string
When dealing with borked device numbers,
FMP24 -i"serial string" is probably the most reliable way to fire up and use the correct dongle. Definitely useful when you have more than one RTL SDR dongle plugged in.
If you're using the supplied batch files, like FMP24-CC.bat, just edit with notepad and change the value after
-i to what works. I use
-i0, which should always find an unused/available RTL SDR and grab it.