- ensure contributors are not listed multiple times

- provide `EXTERNAL_CONTRIBUTORS` template, not including the lib owner

dist/index.js

@@ -2787,12 +2787,14 @@ function buildChangelog(diffInfo, origPrs, options) {
     }
     core.info(`✒️ Wrote ${ignoredPrs.length} ignored pull requests down`);
     // collect all contributors
-    let contributors = [];
+    const contributorsSet = new Set();
     for (const pr of prs) {
-        contributors = contributors.concat(`@${pr.author}`);
+        contributorsSet.add(`@${pr.author}`);
     }
-    const contributorsString = contributors.join(', ');
-    core.setOutput('contributors', JSON.stringify(contributors));
+    const contributorsArray = Array.from(contributorsSet);
+    const contributorsString = contributorsArray.join(', ');
+    const externalContributorString = contributorsArray.filter(value => value !== options.owner).join(', ');
+    core.setOutput('contributors', JSON.stringify(contributorsSet));
     // fill template
     const placeholderMap = new Map();
     placeholderMap.set('CHANGELOG', changelog);
@@ -2801,6 +2803,7 @@ function buildChangelog(diffInfo, origPrs, options) {
     placeholderMap.set('IGNORED', changelogIgnored);
     // fill special collected contributors
     placeholderMap.set('CONTRIBUTORS', contributorsString);
+    placeholderMap.set('EXTERNAL_CONTRIBUTORS', externalContributorString);
     // fill other placeholders
     placeholderMap.set('CATEGORIZED_COUNT', categorizedPrs.length.toString());
     placeholderMap.set('UNCATEGORIZED_COUNT', uncategorizedPrs.length.toString());