Suspension Noise problems of the 2009 Mercury Sable

I now have 75,000 miles on the vehicle, but at around the 15,000 mile point the driver's side front strut went out followed by the passenger front strut 4,000 miles later. In total both front struts have been replaced three times (the original and two replacement sets) and the driver's side front strut is again making a clunking sound, same as before. The dealer has replaced the original and first replacement set at their cost but the last set was at my cost. With the struts showing signs of needing replacement again I don't feel that I should be funding for the cost, there is definitely a defect in the struts if that many became defective in that short of a time period. The vehicle is driven on good roads and not abused. Since this is part of the suspension system it can/will affect driving safety and tire wear.