Two Target boosters will both be at 60% effectiveness (120% total).
Three Target boosters will be at 36% effectiveness (118% total).
This is actually worse then only having two. Perhaps make it 45% for 3 (135% total).
Similarly 4 target boosters yields 21.6% each, (86.4% total).
Even if the increase is very small, adding a duplicate module should still give some benefit.