Prevent multiple PackagingSpecs having same installation path

This fixes a bug that different PackagingSpecs having the same
installation path were silently allowed. Previously, a PackagingSpec
that comes the first for the given installation path won, effectively
eclipsing other PackagingSpecs destined for the same installation path.

Bug: 335506668
Test: go test ./...
Change-Id: Ia36f656e8364f95c4be78fff6e9dc16966307526

filesystem/filesystem_test.go

@@ -442,3 +442,26 @@ func TestInconsistentPartitionTypesInDefaults(t *testing.T) {
 		}
 	`)
 }
+
+func TestPreventDuplicatedEntries(t *testing.T) {
+	fixture.ExtendWithErrorHandler(android.FixtureExpectsOneErrorPattern(
+		"packaging conflict at")).
+		RunTestWithBp(t, `
+		android_filesystem {
+			name: "fs",
+			deps: [
+				"foo",
+				"foo_dup",
+			],
+		}
+
+		cc_binary {
+			name: "foo",
+		}
+
+		cc_binary {
+			name: "foo_dup",
+			stem: "foo",
+		}
+	`)
+}