add @primary_extension_index to ensure we don't have duped primary exts

lib/linguist/language.rb

@@ -15,8 +15,10 @@ module Linguist
     @index           = {}
     @name_index      = {}
     @alias_index     = {}
-    @extension_index = Hash.new { |h,k| h[k] = [] }
-    @filename_index  = Hash.new { |h,k| h[k] = [] }
+
+    @extension_index          = Hash.new { |h,k| h[k] = [] }
+    @filename_index           = Hash.new { |h,k| h[k] = [] }
+    @primary_extension_index  = {}
     # Valid Languages types
     TYPES = [:data, :markup, :programming]
@@ -63,6 +65,12 @@ module Linguist
         @extension_index[extension] << language
       end
+      if @primary_extension_index.key?(language.primary_extension)
+        raise ArgumentError, "Duplicate primary extension: #{language.primary_extension}"
+      end
+
+      @primary_extension_index[language.primary_extension] = language
+
       language.filenames.each do |filename|
         @filename_index[filename] << language
       end