Flysafair overbooking flights

Bad experience with flysafair. Prechecked in and booked seats previous day already. We had 1 suitcase that had to be checked in for our flight 14h25. We stood in the queues to check in our 1 bag. When we got to the front they told us that we 3 minutes late to check in luggage but we were in the queue.

Because we had already our boarding pass I boarded and husband stayed behind to be on a standby flight. I was still in time to baord about 8 people behind me. Very surprised when I got to my seat that someone else was already comfortable in my husband seat. How is that possible? He eventually got a flight only at 19h00. I am stuck at OR Tambo and he was stuck at Cape Town international. If we didn't prechecked in I could understand but we did previous day. What if we did not had a bag to check in he could also walk straight through like me and I wonder what would have happened when he got to his seat with his boarding pass and someone else was already checked in same seat.