diff --git a/tools/gn/function_forward_variables_from.cc b/tools/gn/function_forward_variables_from.cc
index dd3629b..8f3b90d 100644
--- a/tools/gn/function_forward_variables_from.cc
+++ b/tools/gn/function_forward_variables_from.cc
@@ -54,6 +54,18 @@
         return;
       }
 
+      // Don't allow clobbering existing values.
+      const Value* existing_value = dest->GetValue(storage_key);
+      if (existing_value) {
+        *err = Err(cur, "Clobbering existing value.",
+            "The current scope already defines a value \"" +
+             cur.string_value() + "\".\nforward_variables_from() won't clobber "
+             "existing values. If you want to\nmerge lists, you'll need to "
+             "do this explicitly.");
+        err->AppendSubErr(Err(*existing_value, "value being clobbered."));
+        return;
+      }
+
       // Keep the origin information from the original value. The normal
       // usage is for this to be used in a template, and if there's an error,
       // the user expects to see the line where they set the variable
diff --git a/tools/gn/function_forward_variables_from_unittest.cc b/tools/gn/function_forward_variables_from_unittest.cc
index e5137a4..2e6f5a2 100644
--- a/tools/gn/function_forward_variables_from_unittest.cc
+++ b/tools/gn/function_forward_variables_from_unittest.cc
@@ -8,28 +8,46 @@
 
 TEST(FunctionForwardVariablesFrom, List) {
   Scheduler scheduler;
-  TestWithScope setup;
-
-  // Defines a template and copy the two x and y, and z values out.
-  TestParseInput input(
-    "template(\"a\") {\n"
-    "  forward_variables_from(invoker, [\"x\", \"y\", \"z\"])\n"
-    "  assert(!defined(z))\n"  // "z" should still be undefined.
-    "  print(\"$target_name, $x, $y\")\n"
-    "}\n"
-    "a(\"target\") {\n"
-    "  x = 1\n"
-    "  y = 2\n"
-    "}\n");
-
-  ASSERT_FALSE(input.has_error());
-
   Err err;
-  input.parsed()->Execute(setup.scope(), &err);
-  ASSERT_FALSE(err.has_error()) << err.message();
+  std::string program =
+      "template(\"a\") {\n"
+      "  forward_variables_from(invoker, [\"x\", \"y\", \"z\"])\n"
+      "  assert(!defined(z))\n"  // "z" should still be undefined.
+      "  print(\"$target_name, $x, $y\")\n"
+      "}\n"
+      "a(\"target\") {\n"
+      "  x = 1\n"
+      "  y = 2\n"
+      "}\n";
 
-  EXPECT_EQ("target, 1, 2\n", setup.print_output());
-  setup.print_output().clear();
+  {
+    TestWithScope setup;
+
+    // Defines a template and copy the two x and y, and z values out.
+    TestParseInput input(program);
+    ASSERT_FALSE(input.has_error());
+
+    input.parsed()->Execute(setup.scope(), &err);
+    ASSERT_FALSE(err.has_error()) << err.message();
+
+    EXPECT_EQ("target, 1, 2\n", setup.print_output());
+    setup.print_output().clear();
+  }
+
+  {
+    TestWithScope setup;
+
+    // Test that the same input but forwarding a variable with the name of
+    // something in the given scope throws an error rather than clobbering it.
+    // This uses the same known-good program as before, but adds another
+    // variable in the scope before it.
+    TestParseInput clobber("x = 1\n" + program);
+    ASSERT_FALSE(clobber.has_error());
+
+    clobber.parsed()->Execute(setup.scope(), &err);
+    ASSERT_TRUE(err.has_error());  // Should thow a clobber error.
+    EXPECT_EQ("Clobbering existing value.", err.message());
+  }
 }
 
 TEST(FunctionForwardVariablesFrom, ListWithExclusion) {