diff --git a/lib/github_changelog_generator/generator/generator.rb b/lib/github_changelog_generator/generator/generator.rb
index 86ebdd7af..972afd0dc 100644
--- a/lib/github_changelog_generator/generator/generator.rb
+++ b/lib/github_changelog_generator/generator/generator.rb
@@ -25,6 +25,11 @@ class ChangelogGeneratorError < StandardError
   # @see GitHubChangelogGenerator::Section
   class Generator
     attr_accessor :options, :filtered_tags, :tag_section_mapping, :sorted_tags
+    CREDIT_LINE = <<~CREDIT
+      \\* *This Changelog was automatically generated \
+      by [github_changelog_generator]\
+      (https://github.com/github-changelog-generator/github-changelog-generator)*
+    CREDIT
 
     # A Generator responsible for all logic, related with changelog generation from ready-to-parse issues
     #
@@ -46,22 +51,14 @@ def compound_changelog
       fetch_and_filter_tags
       fetch_issues_and_pr
 
-      log = ""
-      log += @options[:frontmatter] if @options[:frontmatter]
-      log += "#{options[:header]}\n\n"
-
-      log += if @options[:unreleased_only]
-               generate_entry_between_tags(@filtered_tags[0], nil)
-             else
-               generate_entries_for_all_tags
-             end
-
+      log = if @options[:unreleased_only]
+              generate_entry_between_tags(@filtered_tags[0], nil)
+            else
+              generate_entries_for_all_tags
+            end
       log += File.read(@options[:base]) if File.file?(@options[:base])
-
-      credit_line = "\n\n\\* *This Changelog was automatically generated by [github_changelog_generator](https://github.com/github-changelog-generator/github-changelog-generator)*"
-      log.gsub!(/#{credit_line}(\n)?/, "") # Remove old credit lines
-      log += "#{credit_line}\n"
-
+      log = remove_old_fixed_string(log)
+      log = insert_fixed_string(log)
       @log = log
     end
 
@@ -151,5 +148,26 @@ def fetch_issues_and_pr
       add_first_occurring_tag_to_prs(@sorted_tags, @pull_requests)
       nil
     end
+
+    # Remove the previously assigned fixed message.
+    # @param log [String] Old lines are fixed
+    def remove_old_fixed_string(log)
+      log.gsub!(/#{Regexp.escape(@options[:frontmatter])}/, "") if @options[:frontmatter]
+      log.gsub!(/#{Regexp.escape(@options[:header])}\n{,2}/, "")
+      log.gsub!(/\n{,2}#{Regexp.escape(CREDIT_LINE)}/, "") # Remove old credit lines
+      log
+    end
+
+    # Add template messages to given string. Previously added
+    # messages of the same wording are removed.
+    # @param log [String]
+    def insert_fixed_string(log)
+      ins = ""
+      ins += @options[:frontmatter] if @options[:frontmatter]
+      ins += "#{@options[:header]}\n\n"
+      log.insert(0, ins)
+      log += "\n\n#{CREDIT_LINE}"
+      log
+    end
   end
 end
diff --git a/spec/unit/generator/generator_spec.rb b/spec/unit/generator/generator_spec.rb
new file mode 100644
index 000000000..7573c44fa
--- /dev/null
+++ b/spec/unit/generator/generator_spec.rb
@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+require "github_changelog_generator/generator/generator"
+
+RSpec.describe GitHubChangelogGenerator::Generator do
+  let(:header) { "# Changelog" }
+  let(:generator) { described_class.new({ header: header }) }
+  let(:content) do
+    <<~'BASE'
+      ## [1.3.10](https://github.com/xxx/yyy/tree/1.3.10) (2015-03-18)
+
+      [Full Changelog](https://github.com/xxx/yyy/compare/1.3.9...1.3.10)
+
+      **Fixed bugs:**
+
+
+    BASE
+  end
+  let(:footer) do
+    <<~CREDIT
+      \\* *This Changelog was automatically generated \
+      by [github_changelog_generator]\
+      (https://github.com/github-changelog-generator/github-changelog-generator)*
+    CREDIT
+  end
+
+  context "when the given base file has previously appended template messages" do
+    describe "#remove_old_fixed_string" do
+      it "removes old template headers and footers" do
+        log = +"#{header}\n\n#{header}\n#{header}#{content}\n\n#{footer}\n#{footer}#{footer}"
+
+        expect(generator.send(:remove_old_fixed_string, log)).to eq content
+      end
+    end
+  end
+
+  context "when plain contents string was given" do
+    describe "#insert_fixed_string" do
+      it "append template messages at header and footer" do
+        log = String.new(content)
+        ans = "#{header}\n\n#{content}\n\n#{footer}"
+
+        expect(generator.send(:insert_fixed_string, log)).to eq ans
+      end
+    end
+  end
+end