Freeze configuration defaults & duplicate in deep_merge_hashes if need be.

lib/jekyll/configuration.rb

@@ -72,7 +72,7 @@ module Jekyll
         'hard_wrap'      => false,
         'footnote_nr'    => 1
       }
-    }].freeze
+    }.map { |k, v| [k, v.freeze] }].freeze
 
     class << self
       # Static: Produce a Configuration ready for use in a Site.

lib/jekyll/utils.rb

@@ -54,6 +54,10 @@ module Jekyll
         target.default_proc = overwrite.default_proc
       end
 
+      target.each do |key, val|
+        target[key] = val.dup if val.frozen? && duplicable?(val)
+      end
+
       target
     end
 
@@ -61,6 +65,15 @@ module Jekyll
       value.is_a?(Hash) || value.is_a?(Drops::Drop)
     end
 
+    def duplicable?(obj)
+      case obj
+      when nil, false, true, Symbol, Numeric
+        false
+      else
+        true
+      end
+    end
+
     # Read array from the supplied hash favouring the singular key
     # and then the plural key, and handling any nil entries.
     #