diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,10 @@
 # Revision history for th-abstraction
 
+## 0.2.7.0 -- 2018-06-17
+* Fix bug in which data family instances with duplicate occurrences of type
+  variables in the left-hand side would have redundant equality constraints
+  in their contexts.
+
 ## 0.2.6.0 -- 2017-09-04
 * Fix bug in which `applySubstitution` and `freeVariables` would ignore
   type variables in the kinds of type variable binders.
diff --git a/src/Language/Haskell/TH/Datatype.hs b/src/Language/Haskell/TH/Datatype.hs
--- a/src/Language/Haskell/TH/Datatype.hs
+++ b/src/Language/Haskell/TH/Datatype.hs
@@ -929,17 +929,28 @@
   (Map Name Name, Cxt)
 mergeArguments ns ts = foldr aux (Map.empty, []) (zip ns ts)
   where
-    aux (SigT x _, y) sc = aux (x,y) sc -- learn about kinds??
-    aux (x, SigT y _) sc = aux (x,y) sc
 
     aux (f `AppT` x, g `AppT` y) sc =
       aux (x,y) (aux (f,g) sc)
 
     aux (VarT n,p) (subst, context) =
       case p of
-        VarT m | Map.notMember m subst -> (Map.insert m n subst, context)
+        VarT m | m == n  -> (subst, context)
+                   -- If the two variables are the same, don't bother extending
+                   -- the substitution. (This is purely an optimization.)
+               | Just n' <- Map.lookup m subst
+               , n == n' -> (subst, context)
+                   -- If a variable is already in a substitution and it maps
+                   -- to the variable that we are trying to unify with, then
+                   -- leave the context alone. (Not doing so caused #46.)
+               | Map.notMember m subst -> (Map.insert m n subst, context)
         _ -> (subst, equalPred (VarT n) p : context)
 
+    aux (SigT x _, y) sc = aux (x,y) sc -- learn about kinds??
+    -- This matches *after* VarT so that we can compute a substitution
+    -- that includes the kind signature.
+    aux (x, SigT y _) sc = aux (x,y) sc
+
     aux _ sc = sc
 
 -- | Expand all of the type synonyms in a type.
@@ -1255,6 +1266,10 @@
 
 -- | Compute the type variable substitution that unifies a list of types,
 -- or fail in 'Q'.
+--
+-- All infix issue should be resolved before using 'unifyTypes'
+--
+-- Alpha equivalent quantified types are not unified.
 unifyTypes :: [Type] -> Q (Map Name Type)
 unifyTypes [] = return Map.empty
 unifyTypes (t:ts) =
@@ -1277,20 +1292,23 @@
 
 unify' (VarT n) (VarT m) | n == m = pure Map.empty
 unify' (VarT n) t | n `elem` freeVariables t = Left (VarT n, t)
-                  | otherwise                = pure (Map.singleton n t)
+                  | otherwise                = Right (Map.singleton n t)
 unify' t (VarT n) | n `elem` freeVariables t = Left (VarT n, t)
-                  | otherwise                = pure (Map.singleton n t)
-
-unify' (ConT n) (ConT m) | n == m = pure Map.empty
+                  | otherwise                = Right (Map.singleton n t)
 
 unify' (AppT f1 x1) (AppT f2 x2) =
   do sub1 <- unify' f1 f2
      sub2 <- unify' (applySubstitution sub1 x1) (applySubstitution sub1 x2)
-     return (combineSubstitutions sub1 sub2)
+     Right (combineSubstitutions sub1 sub2)
 
-unify' (TupleT n) (TupleT m) | n == m = pure Map.empty
+-- Doesn't unify kind signatures
+unify' (SigT t _) u = unify' t u
+unify' t (SigT u _) = unify' t u
 
-unify' t u = Left (t,u)
+-- only non-recursive cases should remain at this point
+unify' t u
+  | t == u    = Right Map.empty
+  | otherwise = Left (t,u)
 
 
 -- | Construct an equality constraint. The implementation of 'Pred' varies
diff --git a/test/Main.hs b/test/Main.hs
--- a/test/Main.hs
+++ b/test/Main.hs
@@ -26,10 +26,8 @@
 import           Control.Monad (zipWithM_)
 #endif
 
-#if MIN_VERSION_template_haskell(2,8,0)
 import           Control.Monad (unless)
 import qualified Data.Map as Map
-#endif
 
 #if MIN_VERSION_base(4,7,0)
 import           Data.Type.Equality ((:~:)(..))
@@ -64,6 +62,7 @@
      famLocalDecTest1
      famLocalDecTest2
      recordFamTest
+     t46Test
 #endif
      fixityLookupTest
 #if __GLASGOW_HASKELL__ >= 704
@@ -77,6 +76,7 @@
 #if MIN_VERSION_template_haskell(2,8,0)
      kindSubstTest
 #endif
+     regressionTest44
 
 adt1Test :: IO ()
 adt1Test =
@@ -512,6 +512,16 @@
 recordFamTest =
   $(do info <- reifyRecord 'famRec1
        validateCI info gadtRecFamCI)
+
+t46Test :: IO ()
+t46Test =
+  $(do info <- reifyDatatype 'MkT46
+       case info of
+         DatatypeInfo { datatypeCons = [ConstructorInfo { constructorContext = ctxt }]} ->
+           unless (null ctxt) (fail "regression test for ticket #46 failed")
+         _ -> fail "T46 should have exactly one constructor"
+       [| return () |])
+
 #endif
 
 fixityLookupTest :: IO ()
@@ -606,3 +616,10 @@
        checkFreeVars substTy [k2]
        [| return () |])
 #endif
+
+regressionTest44 :: IO ()
+regressionTest44 =
+  $(do intToInt <- [t| Int -> Int |]
+       unified  <- unifyTypes [intToInt, intToInt]
+       unless (Map.null unified) (fail "regression test for ticket #44 failed")
+       [| return () |])
diff --git a/test/Types.hs b/test/Types.hs
--- a/test/Types.hs
+++ b/test/Types.hs
@@ -95,6 +95,9 @@
 
 data family FamLocalDec1 a
 data family FamLocalDec2 a b c
+
+data family   T46 a b c
+data instance T46 (f (p :: *)) (f p) q = MkT46 q
 #endif
 
 #if __GLASGOW_HASKELL__ >= 704
diff --git a/th-abstraction.cabal b/th-abstraction.cabal
--- a/th-abstraction.cabal
+++ b/th-abstraction.cabal
@@ -1,5 +1,5 @@
 name:                th-abstraction
-version:             0.2.6.0
+version:             0.2.7.0
 synopsis:            Nicer interface for reified information about data types
 description:         This package normalizes variations in the interface for
                      inspecting datatype information via Template Haskell
@@ -17,7 +17,7 @@
 build-type:          Simple
 extra-source-files:  ChangeLog.md README.md
 cabal-version:       >=1.10
-tested-with:         GHC==8.0.2, GHC==7.10.3, GHC==7.8.4, GHC==7.6.3, GHC==7.4.2, GHC==7.2.2, GHC==7.0.4
+tested-with:         GHC==8.4.3, GHC==8.2.2, GHC==8.0.2, GHC==7.10.3, GHC==7.8.4, GHC==7.6.3, GHC==7.4.2, GHC==7.2.2, GHC==7.0.4
 
 source-repository head
   type: git
@@ -28,7 +28,7 @@
   other-modules:       Language.Haskell.TH.Datatype.Internal
   build-depends:       base             >=4.3   && <5,
                        ghc-prim,
-                       template-haskell >=2.5   && <2.13,
+                       template-haskell >=2.5   && <2.14,
                        containers       >=0.4   && <0.6
   hs-source-dirs:      src
   default-language:    Haskell2010