The MAS speed readout can be doubled if the rotation sensor puts out two pulses per rotation instead of the usual one. With optical detection, that could happen if the optical mark on the rotor is partially worn off, not well-defined, or actually two separate marks. This is rare, at least in my experience.

If you see this effect often, or all the time, there may be a problem with the MAS speed sensor/readout electronics.

One other possibility: If all spectral features are reduced to half of the expected frequency scale (incorrect chemical shifts differences, CSA span, and resonance offset), then there is a problem with the digitization or FFT processing mode.

Most likely you have a problem reading the right spinning frequency. The best thing to do is to take the tachometer signal and display it on the scope. The tachometer signal should look like a series of square pulses. If it looks somewhat different that means that something is wrong, and the signal gives you an idea what exactly went wrong. For instance, you can literally count how many pulses per unit of time you have and convert it into frequency. As was mentioned earlier a worn off mark, which in reality consists of two marks could cause such a problem, and the tachometer signal should show you this as two pulses instead of one.

The other simple test to do for distinguishing between 6 and 12 kHz is to record the CP in H-15N signal as a function of say H rf strength. You must see two best match rf fields, which will be separated either by 12 or by 24 kHz. But of course this does not give you any idea of what happened to the spinning speed reading.