fix: prevent duplicate headers from being added (conventional-changel…

…og#305)

lib/lifecycles/changelog.js

@@ -5,6 +5,7 @@ const conventionalChangelog = require('conventional-changelog')
 const fs = require('fs')
 const runLifecycleScript = require('../run-lifecycle-script')
 const writeFile = require('../write-file')
+const START_OF_LAST_RELEASE_PATTERN = /(^##? (?!Change Log$)|<a name=)/m
 
 module.exports = function (args, newVersion) {
   if (args.skip.changelog) return Promise.resolve()
@@ -22,9 +23,10 @@ function outputChangelog (args, newVersion) {
     createIfMissing(args)
     var header = '# Change Log\n\nAll notable changes to this project will be documented in this file. See [standard-version](https://github.com/conventional-changelog/standard-version) for commit guidelines.\n'
     var oldContent = args.dryRun ? '' : fs.readFileSync(args.infile, 'utf-8')
+    var oldContentStart = oldContent.search(START_OF_LAST_RELEASE_PATTERN)
     // find the position of the last release and remove header:
-    if (oldContent.indexOf('<a name=') !== -1) {
-      oldContent = oldContent.substring(oldContent.indexOf('<a name='))
+    if (oldContentStart !== -1) {
+      oldContent = oldContent.substring(oldContentStart)
     }
     var content = ''
     var context