Deduplicate packages in the install plan after the solver removes qua…

…lifiers.

Fixes #4026

cabal-install/Distribution/Solver/Modular.hs

@@ -9,8 +9,14 @@ module Distribution.Solver.Modular
 -- and finally, we have to convert back the resulting install
 -- plan.
 
+import Data.Function
+         ( on )
+import Data.List
+         ( nubBy )
 import Data.Map as M
          ( fromListWith )
+import Distribution.Compat.Graph
+         ( IsNode(..) )
 import Distribution.Solver.Modular.Assignment
          ( toCPs )
 import Distribution.Solver.Modular.ConfiguredConversion
@@ -44,8 +50,12 @@ modularResolver sc (Platform arch os) cinfo iidx sidx pkgConfigDB pprefs pcs pns
         where
           pair lpc = (pcName $ unlabelPackageConstraint lpc, [lpc])
 
-      -- Results have to be converted into an install plan.
-      postprocess a rdm = map (convCP iidx sidx) (toCPs a rdm)
+      -- Results have to be converted into an install plan. 'convCP' removes
+      -- package qualifiers, which means that linked packages become duplicates
+      -- and can be removed.
+      -- TODO: Use ordNubBy instead of nubBy.
+      postprocess a rdm = nubBy ((==) `on` nodeKey) $
+                          map (convCP iidx sidx) (toCPs a rdm)
 
       -- Helper function to extract the PN from a constraint.
       pcName :: PackageConstraint -> PN