Fix update-report migration.

Library/Homebrew/cmd/update-report.rb

@@ -198,6 +198,8 @@ module Homebrew
       next unless /^(?<prefix>[^\.]+[^\-])\-(?<suffix>[^\-].*)/ =~ child.basename.to_s
       target = HOMEBREW_CACHE/"#{prefix}--#{suffix}"
 
+      next if suffix.include?("--") && !suffix.start_with?("patch")
+
       if target.exist?
         begin
           FileUtils.rm_rf child

Library/Homebrew/test/cmd/update-report_spec.rb

@@ -16,6 +16,32 @@ describe "brew update-report" do
       expect(renamed_cache_file).to exist
     end
 
+    context "when the formula name contains dashes" do
+      let(:legacy_cache_file) { HOMEBREW_CACHE/"foo-bar-1.2.3.tar.gz" }
+      let(:renamed_cache_file) { HOMEBREW_CACHE/"foo-bar--1.2.3.tar.gz" }
+
+      it "does not introduce extra double dashes when called multiple times" do
+        Homebrew.migrate_cache_entries_to_double_dashes(Version.new("1.7.1"))
+        Homebrew.migrate_cache_entries_to_double_dashes(Version.new("1.7.1"))
+
+        expect(legacy_cache_file).not_to exist
+        expect(renamed_cache_file).to exist
+      end
+    end
+
+    context "when the file is a patch and the formula name contains dashes" do
+      let(:legacy_cache_file) { HOMEBREW_CACHE/"foo-bar-patch--1.2.3.tar.gz" }
+      let(:renamed_cache_file) { HOMEBREW_CACHE/"foo-bar--patch--1.2.3.tar.gz" }
+
+      it "does not introduce extra double dashes when called multiple times" do
+        Homebrew.migrate_cache_entries_to_double_dashes(Version.new("1.7.1"))
+        Homebrew.migrate_cache_entries_to_double_dashes(Version.new("1.7.1"))
+
+        expect(legacy_cache_file).not_to exist
+        expect(renamed_cache_file).to exist
+      end
+    end
+
     it "does not move files if upgrading from > 1.7.1" do
       Homebrew.migrate_cache_entries_to_double_dashes(Version.new("1.7.2"))