Quote from One:
TNET,
I read your posts quickly and am still unsure whether you:
a. modified your existing order from to purchase at 9 (to purchase at 5).
b. entered a new order to purchase at 5.
If the answer is a) and you had two fills, the error is in the software. While possible, I will note that every day I have working orders with modified prices filled in and ready to transmit, but have never had two fills (whether or not the order was filled at the original price, and regardless of timing of transmitting the modified order).
If the answer is b) it seems there are two possibilities: the new order was transmitted without you hitting "T", which seems unlikely but would be a IB's responsiblity; or the order was transmitted (we can all forget clicking a mouse while trading) and either TWS did not report the fill that was reported to them by the exchange, or the exchange did not report the fill immediately (happens), both of which are your responsibility since you placed the order.
Please let us know which of these scenarios or other is correct, and how your discussions with IB are resolved.
By the way, a good rule of thumb is to liquidate any position in error, whether one lot or 250. In every discrepancy I have found brokers have based compensation on the price at the time the error was discovered.
Good luck.