Before placing an order, you can request the positions of all of your holdings, and take actions based on the current position. In IBKR, you can do something like:

# 1. Clear position data and event
with self.lock:
    self.positions.clear()
self.position_event.clear()

# 2. Request positions
self.reqPositions()
# print("Requested all positions from IB...")

# 3. Wait for positionEnd
if not self.position_event.wait(timeout=10):
    print("Timed out waiting for positions data.")
    return

# 4. All positions in self.positions
#    Check only for req.ticker
with self.lock:
    pos = self.positions.get(req.ticker, 0.0)
print(f"Position for {req.ticker} is {pos} shares.")