diff --git a/cabal-install/src/Distribution/Client/Dependency.hs b/cabal-install/src/Distribution/Client/Dependency.hs
index a65c41cb046..bbbb1021f68 100644
--- a/cabal-install/src/Distribution/Client/Dependency.hs
+++ b/cabal-install/src/Distribution/Client/Dependency.hs
@@ -993,7 +993,7 @@ planPackagesProblems platform cinfo pkgs =
   , not (null packageProblems)
   ]
     ++ [ DuplicatePackageSolverId (Graph.nodeKey aDup) dups
-       | dups <- duplicatesBy (comparing Graph.nodeKey) pkgs
+       | dups <- fmap toList $ duplicatesBy (comparing Graph.nodeKey) pkgs
        , aDup <- case dups of
           [] -> []
           (ad : _) -> [ad]
@@ -1055,7 +1055,7 @@ configuredPackageProblems
          | pkgs <-
             CD.nonSetupDeps
               ( fmap
-                  (duplicatesBy (comparing packageName))
+                  (fmap toList . duplicatesBy (comparing packageName))
                   specifiedDeps1
               )
          ]
diff --git a/cabal-install/src/Distribution/Client/ProjectPlanning.hs b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
index 685e46dfa77..2339b91f147 100644
--- a/cabal-install/src/Distribution/Client/ProjectPlanning.hs
+++ b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
@@ -109,6 +109,7 @@ import Text.PrettyPrint
   , text
   , vcat
   , ($$)
+  , ($+$)
   )
 import Prelude ()
 
@@ -137,7 +138,7 @@ import Distribution.Client.SetupWrapper
 import Distribution.Client.Store
 import Distribution.Client.Targets (userToPackageConstraint)
 import Distribution.Client.Types
-import Distribution.Client.Utils (concatMapM, incVersion)
+import Distribution.Client.Utils (concatMapM, duplicatesBy, incVersion)
 
 import qualified Distribution.Client.BuildReports.Storage as BuildReports
 import qualified Distribution.Client.IndexUtils as IndexUtils
@@ -451,13 +452,29 @@ rebuildProjectConfig
             createDirectoryIfMissingVerbose verbosity True distDirectory
             createDirectoryIfMissingVerbose verbosity True distProjectCacheDirectory
 
-          fetchAndReadSourcePackages
-            verbosity
-            distDirLayout
-            compiler
-            projectConfigShared
-            projectConfigBuildOnly
-            pkgLocations
+          sourcePackages <-
+            fetchAndReadSourcePackages
+              verbosity
+              distDirLayout
+              compiler
+              projectConfigShared
+              projectConfigBuildOnly
+              pkgLocations
+
+          case duplicatesBy (comparing srcpkgPackageId) [pkg | SpecificSourcePackage pkg <- sourcePackages] of
+            [] -> return ()
+            duplicateSourcePkgs ->
+              liftIO $
+                noticeDoc verbosity $
+                  vcat
+                    [ text "cabal project has multiple sources for"
+                      <+> (pretty (srcpkgPackageId (head dupeGroup)) <> text ":")
+                      $+$ Disp.nest 2 (vcat [pretty (srcpkgSource srcpkg) | srcpkg <- toList dupeGroup])
+                      $+$ text "the choice of source that will be used is undefined."
+                    | dupeGroup <- duplicateSourcePkgs
+                    ]
+
+          return sourcePackages
 
       informAboutConfigFiles projectConfig = do
         cwd <- getCurrentDirectory
diff --git a/cabal-install/src/Distribution/Client/Types/PackageLocation.hs b/cabal-install/src/Distribution/Client/Types/PackageLocation.hs
index 9a0537aae7a..9dd00c40080 100644
--- a/cabal-install/src/Distribution/Client/Types/PackageLocation.hs
+++ b/cabal-install/src/Distribution/Client/Types/PackageLocation.hs
@@ -16,7 +16,8 @@ import Network.URI (URI)
 import Distribution.Types.PackageId (PackageId)
 
 import Distribution.Client.Types.Repo
-import Distribution.Client.Types.SourceRepo (SourceRepoMaybe)
+import Distribution.Client.Types.SourceRepo (SourceRepoMaybe, SourceRepositoryPackage (..))
+import Distribution.Pretty
 import Distribution.Solver.Types.SourcePackage (SourcePackage)
 
 type UnresolvedPkgLoc = PackageLocation (Maybe FilePath)
@@ -39,6 +40,14 @@ data PackageLocation local
     RemoteSourceRepoPackage SourceRepoMaybe local
   deriving (Show, Functor, Eq, Ord, Generic)
 
+instance Pretty (PackageLocation local) where
+  pretty (LocalUnpackedPackage fp) = showFilePath fp
+  pretty (LocalTarballPackage fp) = showFilePath fp
+  pretty (RemoteTarballPackage uri _) = showToken $ show uri
+  pretty (RepoTarballPackage repo pid _) = pretty pid <> showToken "@" <> pretty (repoName repo)
+  pretty (RemoteSourceRepoPackage sourceRepo _) =
+    pretty (srpType sourceRepo) <+> showToken (srpLocation sourceRepo)
+
 instance Binary local => Binary (PackageLocation local)
 instance Structured local => Structured (PackageLocation local)
 
diff --git a/cabal-install/src/Distribution/Client/Utils.hs b/cabal-install/src/Distribution/Client/Utils.hs
index 07db71a9f9d..92e12e8fcc6 100644
--- a/cabal-install/src/Distribution/Client/Utils.hs
+++ b/cabal-install/src/Distribution/Client/Utils.hs
@@ -140,18 +140,18 @@ mergeBy cmp = merge
 
 data MergeResult a b = OnlyInLeft a | InBoth a b | OnlyInRight b
 
-duplicates :: Ord a => [a] -> [[a]]
+duplicates :: Ord a => [a] -> [NonEmpty a]
 duplicates = duplicatesBy compare
 
-duplicatesBy :: forall a. (a -> a -> Ordering) -> [a] -> [[a]]
-duplicatesBy cmp = filter moreThanOne . groupBy eq . sortBy cmp
+duplicatesBy :: forall a. (a -> a -> Ordering) -> [a] -> [NonEmpty a]
+duplicatesBy cmp = mapMaybe moreThanOne . groupBy eq . sortBy cmp
   where
     eq :: a -> a -> Bool
     eq a b = case cmp a b of
       EQ -> True
       _ -> False
-    moreThanOne (_ : _ : _) = True
-    moreThanOne _ = False
+    moreThanOne (x : xs@(_ : _)) = Just (x :| xs)
+    moreThanOne _ = Nothing
 
 -- | Like 'removeFile', but does not throw an exception when the file does not
 -- exist.
diff --git a/cabal-testsuite/PackageTests/ProjectDupPkg/cabal.out b/cabal-testsuite/PackageTests/ProjectDupPkg/cabal.out
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/cabal-testsuite/PackageTests/ProjectDupPkg/cabal.project b/cabal-testsuite/PackageTests/ProjectDupPkg/cabal.project
new file mode 100644
index 00000000000..2f191f96e7e
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ProjectDupPkg/cabal.project
@@ -0,0 +1 @@
+packages: pkg-one, pkg-two
diff --git a/cabal-testsuite/PackageTests/ProjectDupPkg/cabal.test.hs b/cabal-testsuite/PackageTests/ProjectDupPkg/cabal.test.hs
new file mode 100644
index 00000000000..534ef613f3a
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ProjectDupPkg/cabal.test.hs
@@ -0,0 +1,21 @@
+import Test.Cabal.Prelude
+import Data.List (isInfixOf)
+
+-- output contains filepaths into /tmp, so we only match parts of the output
+main = cabalTest . recordMode DoNotRecord $ do
+      liftIO $ skipIfWindows "\\r\\n confused with \\n"
+
+      let msg = unlines
+            [ "cabal project has multiple sources for pkg-one-0.1:"
+            , "  .*/pkg-one"
+            , "  .*/pkg-two"
+            , "the choice of source that will be used is undefined."
+            ]
+
+      normal <- cabal' "configure" ["-v1", "pkg-one"]
+      assertOutputMatches msg normal
+
+      quiet <- cabal' "configure" ["-v0", "pkg-one"]
+      assertOutputDoesNotMatch msg quiet
+
+      return ()
diff --git a/cabal-testsuite/PackageTests/ProjectDupPkg/pkg-one/Foo.hs b/cabal-testsuite/PackageTests/ProjectDupPkg/pkg-one/Foo.hs
new file mode 100644
index 00000000000..8a39fe134cf
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ProjectDupPkg/pkg-one/Foo.hs
@@ -0,0 +1,4 @@
+module Foo where
+
+a :: Int
+a = 42
diff --git a/cabal-testsuite/PackageTests/ProjectDupPkg/pkg-one/pkg-one.cabal b/cabal-testsuite/PackageTests/ProjectDupPkg/pkg-one/pkg-one.cabal
new file mode 100644
index 00000000000..3e17e5074d7
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ProjectDupPkg/pkg-one/pkg-one.cabal
@@ -0,0 +1,9 @@
+name:          pkg-one
+version:       0.1
+license:       BSD3
+cabal-version: >=1.2
+build-type:    Simple
+
+library
+  exposed-modules: Foo
+  build-depends:   base
diff --git a/cabal-testsuite/PackageTests/ProjectDupPkg/pkg-two/Bar.hs b/cabal-testsuite/PackageTests/ProjectDupPkg/pkg-two/Bar.hs
new file mode 100644
index 00000000000..870176c2b2d
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ProjectDupPkg/pkg-two/Bar.hs
@@ -0,0 +1,4 @@
+module Bar where
+
+a :: Int
+a = 42
diff --git a/cabal-testsuite/PackageTests/ProjectDupPkg/pkg-two/pkg-one.cabal b/cabal-testsuite/PackageTests/ProjectDupPkg/pkg-two/pkg-one.cabal
new file mode 100644
index 00000000000..5708318ba15
--- /dev/null
+++ b/cabal-testsuite/PackageTests/ProjectDupPkg/pkg-two/pkg-one.cabal
@@ -0,0 +1,9 @@
+name:          pkg-one
+version:       0.1
+license:       BSD3
+cabal-version: >=1.2
+build-type:    Simple
+
+library
+  exposed-modules: Bar
+  build-depends:   base
diff --git a/changelog.d/pr-11487 b/changelog.d/pr-11487
new file mode 100644
index 00000000000..8669f946f69
--- /dev/null
+++ b/changelog.d/pr-11487
@@ -0,0 +1,7 @@
+---
+synopsis: Log if there are multiple local packages with the same ID in a project
+packages: [cabal-install]
+prs: 11487
+---
+
+Log duplicate local packages with the same package identifier, so that the user knows that only one will be picked.