There largest overlap, as far as I can see, is between ghc-mod and hdevtools. Both are proofs of concept. Maybe parts could actually be incorporated into GHCi. hsdev seems to be specialized to source file editing and should remain separate but it can use the other ones as data sources (incl. Cabal and GHCi).
Re: wasted effort. I don't agree. hdevtools was made to make a point that running a process in the background which maintains data and answers queries is much more responsive to the user than otherwise. And it has proven that point. Now, GHCi can learn that lesson and provide something similar, if that indeed belongs to GHCi's scope.
Finally, all this is just speculation since I don't know exactly what the actual overlap among the tools is. It might be smaller than we think.