A note on doubles of groups
  Recently, Rips produced an example of a double of two free groups which has unsolvable generalized word problem. In this paper, we show that Rips's example fits into a large class of doubles of groups, each member of which contains F_2 x F_2 and therefore has unsolvable generalized word problem and is incoherent. 
Nadia Benakli, Oliver T. Dasbach, Yair Glasner, Brian Mangum