Aircraft transponder setup problem

I have a problem with the "Listen to Aircraft Transponders" section. When I run the rtl_test -t test, I get the following message: Kernel driver is active, or device is claimed by second instance of librtlsdr. In the first case, please either detach or blacklist the kernel module (dvb_usb_rtl28xxu), or enable automatic detaching at compile time.
Can you tell me what this means?
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