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If you agreed to the short position, then you gambled and lost. If your broker made the second trade without your permission, then that's a theft/conversion and you can contact a private lawyer and sue or go to arbitration.
I don't know how long you've been trading, but the sort of thing you're describing is extremely typical in the commodity and forex markets. Traders prey on "investors" who don't have sufficient resources to stay in a position if it goes against them. They literally wait for the trades to come through and then they push the bid and ask until the investor is wiped out and has to liquidate.
All perfectly legal, and a total scam, simultaneously. Been going on for at least the last 50 years.
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