Your first reading is always the closest to your actual basal temperature, therefore always measure only once.
Our basal thermometer is very sensitive in order to capture the smallest changes in your temperature. Therefore if you measure several times in a row, your body's temperature will naturally rise (and sometimes fall) so the first temperature reading will be different than the next reading. The body temperature changes with how long you have been awake, moving, sitting still, etc. This is why it's important for you to measure as soon as you wake up, before you snooze, lie in bed for a while, or get out of bed.