Warn for duplicate meta data links

Match order of METADATA_LINK_KEYS to order used by rubygems.org in Links model.
Add missing download_uri key.

Author: etherbob

lib/rubygems/specification_policy.rb

@@ -12,13 +12,14 @@ class Gem::SpecificationPolicy
   VALID_URI_PATTERN = %r{\Ahttps?:\/\/([^\s:@]+:[^\s:@]*@)?[A-Za-z\d\-]+(\.[A-Za-z\d\-]+)+\.?(:\d{1,5})?([\/?]\S*)?\z} # :nodoc:
 
   METADATA_LINK_KEYS = %w[
-    bug_tracker_uri
-    changelog_uri
-    documentation_uri
     homepage_uri
-    mailing_list_uri
+    changelog_uri
     source_code_uri
+    documentation_uri
     wiki_uri
+    mailing_list_uri
+    bug_tracker_uri
+    download_uri
     funding_uri
   ].freeze # :nodoc:
 
@@ -106,6 +107,8 @@ def validate_optional(strict)
 
     validate_removed_attributes
 
+    validate_unique_links
+
     if @warnings > 0
       if strict
         error "specification has warnings"
@@ -501,6 +504,22 @@ def validate_rake_extensions(builder) # :nodoc:
     WARNING
   end
 
+  def validate_unique_links
+    links = @specification.metadata.slice(*METADATA_LINK_KEYS)
+    grouped = links.group_by {|_key, uri| uri }
+    grouped.each do |uri, copies|
+      next unless copies.length > 1
+      keys = copies.map(&:first).join("\n  ")
+      warning <<~WARNING
+        You have specified the uri:
+          #{uri}
+        for all of the following keys:
+          #{keys}
+        Only the first one will be shown on rubygems.org
+      WARNING
+    end
+  end
+
   def warning(statement) # :nodoc:
     @warnings += 1
 

test/rubygems/test_gem_specification.rb

@@ -3644,6 +3644,38 @@ def test_metadata_link_validation_fails
     end
   end
 
+  def test_metadata_link_validation_warns_for_duplicates
+    util_setup_validate
+
+    Dir.chdir @tempdir do
+      @m2 = quick_gem "m", "2" do |s|
+        s.files = %w[lib/code.rb]
+        s.licenses = "BSD-2-Clause"
+        s.metadata = {
+          "source_code_uri" => "http://example.com",
+          "homepage_uri" => "http://example.com",
+          "changelog_uri" => "http://example.com/changelog",
+        }
+      end
+
+      use_ui @ui do
+        @m2.validate
+      end
+
+      expected = <<~EXPECTED
+        #{w}:  You have specified the uri:
+          http://example.com
+        for all of the following keys:
+          homepage_uri
+          source_code_uri
+        Only the first one will be shown on rubygems.org
+        #{w}:  See https://guides.rubygems.org/specification-reference/ for help
+      EXPECTED
+
+      assert_equal expected, @ui.error, "warning"
+    end
+  end
+
   def test_metadata_specs
     @m1 = quick_gem "m", "1" do |s|
       s.files = %w[lib/code.rb]