The etiquette is not to "talk over" someone else, but HF is very different to VHF in dispersion characteristics, in that it is not limited to "line of sight".
HF signals can refract and bounce many times, sometimes all the way round the earth.
This used to be the basis for Ham radio communications, and multi-skip worldwide comms was common during sunspot maxima.
Sometimes you can't hear someone else on the freq , but someone thousands of miles away can hear you and others simultaneously. It is not always predictable, so "listen before transmit" is a good policy.