Fix duplicate mount sources

Fixes #9426

hugolib/filesystems/basefs_test.go

@@ -33,7 +33,6 @@ import (
 	"github.com/gohugoio/hugo/hugofs"
 	"github.com/gohugoio/hugo/hugolib/paths"
 	"github.com/gohugoio/hugo/modules"
-	
 )
 
 func initConfig(fs afero.Fs, cfg config.Provider) error {
@@ -152,7 +151,7 @@ theme = ["atheme"]
 	checkFileCount(bfs.Data.Fs, "", c, 11)       // 7 + 4 themes
 	checkFileCount(bfs.Archetypes.Fs, "", c, 10) // 8 + 2 themes
 	checkFileCount(bfs.Assets.Fs, "", c, 9)
-	checkFileCount(bfs.Work, "", c, 82)
+	checkFileCount(bfs.Work, "", c, 90)
 
 	c.Assert(bfs.IsData(filepath.Join(workingDir, "mydata", "file1.txt")), qt.Equals, true)
 	c.Assert(bfs.IsI18n(filepath.Join(workingDir, "myi18n", "file1.txt")), qt.Equals, true)

hugolib/hugo_modules_test.go

@@ -1134,3 +1134,39 @@ P1: {{ $p1.Title }}|{{ $p1.RelPermalink }}|Filename: {{ $p1.File.Filename }}
 
 	b.AssertFileContent("public/index.html", "P1: Abs|/p1/", "Filename: "+contentFilename)
 }
+
+// Issue 9426
+func TestMountSameSource(t *testing.T) {
+	config := `baseURL = 'https://example.org/'
+languageCode = 'en-us'
+title = 'Hugo GitHub Issue #9426'
+
+disableKinds = ['RSS','sitemap','taxonomy','term']
+
+[[module.mounts]]
+source = "content"
+target = "content"
+
+[[module.mounts]]
+source = "extra-content"
+target = "content/resources-a"
+
+[[module.mounts]]
+source = "extra-content"
+target = "content/resources-b"
+`
+	b := newTestSitesBuilder(t).WithConfigFile("toml", config)
+
+	b.WithContent("p1.md", "")
+
+	b.WithSourceFile(
+		"extra-content/_index.md", "",
+		"extra-content/subdir/_index.md", "",
+		"extra-content/subdir/about.md", "",
+	)
+
+	b.Build(BuildCfg{})
+
+	b.AssertFileContent("public/resources-a/subdir/about/index.html", "Single")
+	b.AssertFileContent("public/resources-b/subdir/about/index.html", "Single")
+}