Fix panic when specifying multiple excludeFiles directives

Fixes #9076

modules/config.go

@@ -15,6 +15,7 @@ package modules
 
 import (
 	"fmt"
+	"path"
 	"path/filepath"
 	"strings"
 
@@ -386,6 +387,11 @@ type Mount struct {
 	ExcludeFiles interface{}
 }
 
+// Used as key to remove duplicates.
+func (m Mount) key() string {
+	return path.Join(m.Lang, m.Source, m.Target)
+}
+
 func (m Mount) Component() string {
 	return strings.Split(m.Target, fileSeparator)[0]
 }