diff --git a/android/fixture.go b/android/fixture.go
index 552c8f569..edbbf08ed 100644
--- a/android/fixture.go
+++ b/android/fixture.go
@@ -237,8 +237,14 @@ func NewFixtureFactory(buildDirSupplier *string, preparers ...FixturePreparer) F
 // A set of mock files to add to the mock file system.
 type MockFS map[string][]byte
 
+// Merge adds the extra entries from the supplied map to this one.
+//
+// Fails if the supplied map files with the same paths are present in both of them.
 func (fs MockFS) Merge(extra map[string][]byte) {
 	for p, c := range extra {
+		if _, ok := fs[p]; ok {
+			panic(fmt.Errorf("attempted to add file %s to the mock filesystem but it already exists", p))
+		}
 		fs[p] = c
 	}
 }
@@ -289,17 +295,40 @@ func FixtureMergeMockFs(mockFS MockFS) FixturePreparer {
 }
 
 // Add a file to the mock filesystem
+//
+// Fail if the filesystem already contains a file with that path, use FixtureOverrideFile instead.
 func FixtureAddFile(path string, contents []byte) FixturePreparer {
 	return FixtureModifyMockFS(func(fs MockFS) {
+		if _, ok := fs[path]; ok {
+			panic(fmt.Errorf("attempted to add file %s to the mock filesystem but it already exists, use FixtureOverride*File instead", path))
+		}
 		fs[path] = contents
 	})
 }
 
 // Add a text file to the mock filesystem
+//
+// Fail if the filesystem already contains a file with that path.
 func FixtureAddTextFile(path string, contents string) FixturePreparer {
 	return FixtureAddFile(path, []byte(contents))
 }
 
+// Override a file in the mock filesystem
+//
+// If the file does not exist this behaves as FixtureAddFile.
+func FixtureOverrideFile(path string, contents []byte) FixturePreparer {
+	return FixtureModifyMockFS(func(fs MockFS) {
+		fs[path] = contents
+	})
+}
+
+// Override a text file in the mock filesystem
+//
+// If the file does not exist this behaves as FixtureAddTextFile.
+func FixtureOverrideTextFile(path string, contents string) FixturePreparer {
+	return FixtureOverrideFile(path, []byte(contents))
+}
+
 // Add the root Android.bp file with the supplied contents.
 func FixtureWithRootAndroidBp(contents string) FixturePreparer {
 	return FixtureAddTextFile("Android.bp", contents)
diff --git a/cc/testing.go b/cc/testing.go
index 6840ef4c5..d8adc6127 100644
--- a/cc/testing.go
+++ b/cc/testing.go
@@ -649,8 +649,11 @@ var PrepareForTestOnWindows = android.GroupFixturePreparers(
 
 // The preparer to include if running a cc related test for linux bionic.
 var PrepareForTestOnLinuxBionic = android.GroupFixturePreparers(
-	// Enable linux bionic.
-	android.FixtureAddTextFile(linuxBionicDefaultsPath, withLinuxBionic()),
+	// Enable linux bionic
+	//
+	// Can be used after PrepareForTestWithCcDefaultModules to override its default behavior of
+	// disabling linux bionic, hence why this uses FixtureOverrideTextFile.
+	android.FixtureOverrideTextFile(linuxBionicDefaultsPath, withLinuxBionic()),
 )
 
 // The preparer to include if running a cc related test for fuchsia.