diagnostics: add formula/cask dup check

Duplicate names across taps are a real pain to deal with, so let's warn the user.

Library/Homebrew/diagnostic.rb

@@ -1040,6 +1040,40 @@ module Homebrew
         end
       end
 
+      def non_core_taps
+        @non_core_taps ||= Tap.installed.reject(&:core_tap?).reject(&:core_cask_tap?)
+      end
+
+      def check_for_duplicate_formulae
+        core_formula_names = CoreTap.instance.formula_names
+        shadowed_formula_full_names = non_core_taps.flat_map do |tap|
+          tap_formula_names = tap.formula_names.map { |s| s.delete_prefix("#{tap.name}/") }
+          (core_formula_names & tap_formula_names).map { |f| "#{tap.name}/#{f}" }
+        end.compact
+        return if shadowed_formula_full_names.empty?
+
+        <<~EOS
+          The following formulae have the same name as core formulae:
+            #{shadowed_formula_full_names.join("\n  ")}
+          You will need to use their full names throughout Homebrew.
+        EOS
+      end
+
+      def check_for_duplicate_casks
+        core_cask_names = CoreCaskTap.instance.cask_tokens
+        shadowed_cask_full_names = non_core_taps.flat_map do |tap|
+          tap_cask_names = tap.cask_tokens.map { |s| s.delete_prefix("#{tap.name}/") }
+          (core_cask_names & tap_cask_names).map { |f| "#{tap.name}/#{f}" }
+        end.compact
+        return if shadowed_cask_full_names.empty?
+
+        <<~EOS
+          The following casks have the same name as core casks:
+            #{shadowed_cask_full_names.join("\n  ")}
+          You will need to use their full names throughout Homebrew.
+        EOS
+      end
+
       def all
         methods.map(&:to_s).grep(/^check_/).sort
       end