Several years ago I was IDB on AS. I had flown ANC-SEA to catch a QX flight to BIL. At the gate in SEA I was told that they had given my seat away since I "never left ANC" according to their computer. Incredible since I was standing at the gate in SEA. They re-booked me SEA-MSP-BIL with MSP-BIL on DL and gave me a $300 voucher. And, I got upgraded on the DL flight, WOOHOO!!
So, Yes, AS has been overbooking flights for many years now.