diff --git a/lib/opal/sprockets/source_map_header_patch.rb b/lib/opal/sprockets/source_map_header_patch.rb
index <HASH>..<HASH> 100644
--- a/lib/opal/sprockets/source_map_header_patch.rb
+++ b/lib/opal/sprockets/source_map_header_patch.rb
@@ -22,7 +22,9 @@ module Opal
 
       def self.inject!(prefix)
         self.prefix = prefix
-        ::Sprockets::Server.send :include, self
+        unless ::Sprockets::Server.ancestors.include?(self)
+          ::Sprockets::Server.send :include, self
+        end
       end
 
       def self.prefix
Guard against multiple calls to the .included hook