diff --git a/src/mime/testdata/test.types.globs2 b/src/mime/testdata/test.types.globs2
index 2e893262ea1..cb5b7899b0d 100644
--- a/src/mime/testdata/test.types.globs2
+++ b/src/mime/testdata/test.types.globs2
@@ -6,3 +6,4 @@
 # mime package test for globs2
 50:document/test:*.t3
 50:example/test:*.t4
+30:example/do-not-use:*.t4
diff --git a/src/mime/type_unix.go b/src/mime/type_unix.go
index 59c99e2c86c..f954bc8a1fb 100644
--- a/src/mime/type_unix.go
+++ b/src/mime/type_unix.go
@@ -49,7 +49,15 @@ func loadMimeGlobsFile(filename string) error {
 			continue
 		}
 
-		setExtensionType(fields[2][1:], fields[1])
+		extension := fields[2][1:]
+		if _, ok := mimeTypes.Load(extension); ok {
+			// We've already seen this extension.
+			// The file is in weight order, so we keep
+			// the first entry that we see.
+			continue
+		}
+
+		setExtensionType(extension, fields[1])
 	}
 	if err := scanner.Err(); err != nil {
 		panic(err)