diff --git a/util/check_filenames.go b/util/check_filenames.go
new file mode 100644
index 0000000..886c3f6
--- /dev/null
+++ b/util/check_filenames.go
@@ -0,0 +1,67 @@
+// Copyright (c) 2018, Google Inc.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+// check_filenames.go checks that filenames are unique. Some of our consumers do
+// not support multiple files with the same name in the same build target, even
+// if they are in different directories.
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+func isSourceFile(in string) bool {
+	return strings.HasSuffix(in, ".c") || strings.HasSuffix(in, ".cc")
+}
+
+func main() {
+	var roots = []string{
+		"crypto",
+		filepath.Join("third_party", "fiat"),
+		"ssl",
+	}
+
+	names := make(map[string]string)
+	var foundCollisions bool
+	for _, root := range roots {
+		err := filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
+			if err != nil {
+				return err
+			}
+			if info.IsDir() {
+				return nil
+			}
+			name := strings.ToLower(info.Name()) // Windows and macOS are case-insensitive.
+			if isSourceFile(name) {
+				if oldPath, ok := names[name]; ok {
+					fmt.Printf("Filename collision found: %s and %s\n", path, oldPath)
+					foundCollisions = true
+				} else {
+					names[name] = path
+				}
+			}
+			return nil
+		})
+		if err != nil {
+			fmt.Printf("Error traversing %s: %s\n", root, err)
+			os.Exit(1)
+		}
+	}
+	if foundCollisions {
+		os.Exit(1)
+	}
+}