It's pretty common - in fact, most questions that are categorized as "sound" are either Doppler effect, general wave questions, or the rare decibel-related intensity question. If it stands out as a topic that you often get wrong, definitely review it sometime today. Basically, don't worry as much about the major Doppler formulas - just focus on the basics: the source must have a relative velocity compared to the observer, perceived frequency increases when they are approaching each other and vice versa, etc.
One common trick that the MCAT likes to use involves asking about perceived wavelength instead of frequency. Just make sure you don't overlook this! If perceived frequency decreases, then perceived wavelength should increase.
Good luck 🙂