diff --git a/cabal-install/src/Distribution/Client/ProjectBuilding.hs b/cabal-install/src/Distribution/Client/ProjectBuilding.hs
index dba81b09b17..cdf269e5bf0 100644
--- a/cabal-install/src/Distribution/Client/ProjectBuilding.hs
+++ b/cabal-install/src/Distribution/Client/ProjectBuilding.hs
@@ -97,6 +97,7 @@ import qualified Data.Map as Map
 import qualified Data.Set as Set
 import qualified Data.ByteString as BS
 import qualified Data.ByteString.Lazy as LBS
+import qualified Data.ByteString.Lazy.Char8 as LBS.Char8
 
 import Control.Exception (Handler (..), SomeAsyncException, assert, catches, handle)
 import System.Directory  (canonicalizePath, createDirectoryIfMissing, doesDirectoryExist, doesFileExist, removeFile, renameDirectory)
@@ -1016,9 +1017,18 @@ buildAndInstallUnpackedPackage verbosity
             -- https://github.com/haskell/cabal/issues/4130
             createDirectoryIfMissingVerbose verbosity True entryDir
 
-            LBS.writeFile
-              (entryDir </> "cabal-hash.txt")
-              (renderPackageHashInputs (packageHashInputs pkgshared pkg))
+            let hashFileName     = entryDir </> "cabal-hash.txt"
+                outPkgHashInputs = renderPackageHashInputs (packageHashInputs pkgshared pkg)
+
+            info verbosity $
+              "creating file with the inputs used to compute the package hash: " ++ hashFileName
+
+            LBS.writeFile hashFileName outPkgHashInputs
+
+            debug verbosity "Package hash inputs:"
+            traverse_
+              (debug verbosity . ("> " ++))
+              (lines $ LBS.Char8.unpack outPkgHashInputs)
 
             -- Ensure that there are no files in `tmpDir`, that are
             -- not in `entryDir`. While this breaks the
diff --git a/cabal-testsuite/PackageTests/Regression/T6906/cabal.config b/cabal-testsuite/PackageTests/Regression/T6906/cabal.config
new file mode 100644
index 00000000000..f32b854bbb8
--- /dev/null
+++ b/cabal-testsuite/PackageTests/Regression/T6906/cabal.config
@@ -0,0 +1,2 @@
+extra-include-dirs: foo
+extra-lib-dirs: bar
diff --git a/cabal-testsuite/PackageTests/Regression/T6906/cabal.project b/cabal-testsuite/PackageTests/Regression/T6906/cabal.project
new file mode 100644
index 00000000000..e6fdbadb439
--- /dev/null
+++ b/cabal-testsuite/PackageTests/Regression/T6906/cabal.project
@@ -0,0 +1 @@
+packages: .
diff --git a/cabal-testsuite/PackageTests/Regression/T6906/cabal.test.hs b/cabal-testsuite/PackageTests/Regression/T6906/cabal.test.hs
new file mode 100644
index 00000000000..7d90659bc44
--- /dev/null
+++ b/cabal-testsuite/PackageTests/Regression/T6906/cabal.test.hs
@@ -0,0 +1,9 @@
+import Test.Cabal.Prelude
+
+main = cabalTest $ do
+   res <- cabalG' ["--config=cabal.config"] "v2-install" ["-v3"]
+   assertOutputContains "creating file with the inputs used to compute the package hash:" res
+   assertOutputContains "extra-lib-dirs: bar" res
+   expectBroken 6906 $ assertOutputDoesNotContain "extra-lib-dirs: bar bar" res
+   assertOutputContains "extra-include-dirs: foo" res
+   expectBroken 6906 $ assertOutputDoesNotContain "extra-include-dirs: foo foo" res
diff --git a/cabal-testsuite/PackageTests/Regression/T6906/issue6906.cabal b/cabal-testsuite/PackageTests/Regression/T6906/issue6906.cabal
new file mode 100644
index 00000000000..f5d89f2932d
--- /dev/null
+++ b/cabal-testsuite/PackageTests/Regression/T6906/issue6906.cabal
@@ -0,0 +1,12 @@
+cabal-version: 3.0
+name:          issue6906
+version:       0
+synopsis:      No duplicate entries for extra-*-dirs config options in the inputs used to compute the package hash
+description:   When informed in the global config
+author:        Javier Neira
+category:      Tests
+maintainer:    atreyu.bbb@gmail.com
+
+executable issue6906
+  build-depends:    base
+  main-is:  main.hs
diff --git a/cabal-testsuite/PackageTests/Regression/T6906/main.hs b/cabal-testsuite/PackageTests/Regression/T6906/main.hs
new file mode 100644
index 00000000000..3b4c8f3b856
--- /dev/null
+++ b/cabal-testsuite/PackageTests/Regression/T6906/main.hs
@@ -0,0 +1 @@
+main = putStrLn "Hello issue6906"