Lumber futures have dropped about 12% since hitting a three-year high two weeks ago, a sign that wood buyers stocked up before duties on Canadian two-by-fours more than doubled this month and that traders are worried about the U.S. housing market. Futures for September delivery fell to around $610 per thousand board feet late Friday and have declined in nine of the past 10 trading sessions.  Canadian producers are charged antidumping and countervailing duties as a result of a decades-long trade dispute between the countries. The duties are separate from broad tariffs ...