Boarding Gate Confusion

Sunday 5th April checked in to board 655am Lanseria-CPT. Unaware another Kulula flight departing 730am & boarding through same gate as 655am flight. My daughter & I would have been last to board as we checked in around 630am. Went to boarding gate, saw plane parked outside door, a lot of passengers seated waiting to board. Assuming passengers were waiting for same flight as us, we sat down & waited as well. After 15 to 20 minutes waiting, saw the plane push back. Went to boarding gate staff and asked which flight it was that was departing as I was expecting that plane to be called for boarding still. was informed it was indeed our flight and that announcements had been made. I told the lady we had been sitting at the gate for 15-20 minutes and no announcements had been made down at the gate for sure. Rushed upstairs to buy 2 tickets for the flight which was then boarding, at a cost of R2100. Unhappy at the lack of clarity on 2 flights going through the same gate within 30 minutes of each other, and why there were no announcements at the gate to alert me. I travel extensively internationally for business, am very fami**** with air travel process. Would like my R2100 back please.