A heart rate (HR) monitor for use in a medical device configurable to measure a patient's ECG such as, for example, a WCD system. Embodiments can include an ECG sensor and a processor configured with classification criteria to classify a received ECG signal into one of a plurality of ECG rhythm types each with a corresponding algorithm to determine the patient's heart rate. The processor uses the classification criteria to identify the ECG signal's type and determine a heart rate of the patient using the corresponding algorithm. The HR monitor is configurable to avoid overcounting of erroneous measurements of R-R intervals that can result from large T-waves and/or bigeminy by comparing the mean of even R-R intervals with the mean of odd R-R intervals. If the means are significantly different double counting or bigeminy is indicated and the HR calculation is adjusted accordingly.