diff --git a/src/build-diff.js b/src/build-diff.js
index 3a53260d..6618d8e7 100644
--- a/src/build-diff.js
+++ b/src/build-diff.js
@@ -1,11 +1,77 @@
+const assert = require("assert");
 const path = require("path");
 const { execSync } = require("child_process");
 const { generateIndex } = require("./build-index");
+const computeShortname = require("./compute-shortname.js");
 
+/**
+ * Spec sort function.
+ *
+ * Lists are sorted by ascending URL.
+ */
 function compareSpecs(a, b) {
   return a.url.localeCompare(b.url);
 }
 
+/**
+ * Return the URL of the provided `specs.json` entry.
+ *
+ * The argument may be a string or a spec object.
+ */
+function getUrl(spec) {
+  return (typeof spec === "string") ? spec.split(' ')[0] : spec.url;
+}
+
+/**
+ * Return true if provided `specs.json` entries have the same URL
+ *
+ * The arguments may be strings or spec objects.
+ */
+function haveSameUrl(s1, s2) {
+  const url1 = getUrl(s1);
+  const url2 = getUrl(s2);
+  return url1 === url2;
+}
+
+/**
+ * Return true if provided `specs.json` entries are distinct specs in the same
+ * series.
+ *
+ * The arguments may be strings or spec objects.
+ */
+function areDistinctSpecsInSameSeries(s1, s2) {
+  const spec1 = (typeof s1 === "string") ? { url: getUrl(s1) } : s1;
+  const spec2 = (typeof s2 === "string") ? { url: getUrl(s2) } : s2;
+  if (spec1.url === spec2.url) {
+    return false;
+  }
+  try {
+    const computed1 = computeShortname(spec1.shortname ?? spec1.url, spec1.forkOf);
+    const computed2 = computeShortname(spec2.shortname ?? spec2.url, spec2.forkOf);
+    const series1 = spec1?.series?.shortname ?? computed1.series.shortname;
+    const series2 = spec2?.series?.shortname ?? computed2.series.shortname;
+    return series1 === series2;
+  }
+  catch {
+    return false;
+  }
+}
+
+/**
+ * Return true if provided spec entries are identical.
+ *
+ * The arguments must be spec objects.
+ */
+function areIdentical(s1, s2) {
+  try {
+    assert.deepStrictEqual(s1, s2);
+    return true;
+  }
+  catch {
+    return false;
+  }
+}
+
 /**
  * Generate the new index file from the given initial list file.
  *
@@ -34,47 +100,55 @@ async function compareIndex(newRef, baseRef, { diffType = "diff", log = console.
   log(`Retrieve index.json at "${baseRef}"... done`);
 
   log(`Compute specs.json diff...`);
-  function hasSameUrl(s1, s2) {
-    const url1 = (typeof s1 === "string") ? s1 : s1.url;
-    const url2 = (typeof s2 === "string") ? s2 : s2.url;
-    return url1 === url2;
-  }
   const diff = {
-    added: newSpecs.filter(spec => !baseSpecs.find(s => hasSameUrl(s, spec))),
-    updated: newSpecs.filter(spec =>
-      !!baseSpecs.find(s => hasSameUrl(s, spec)) &&
+    add: newSpecs.filter(spec => !baseSpecs.find(s => haveSameUrl(s, spec))),
+    update: newSpecs.filter(spec =>
+      !!baseSpecs.find(s => haveSameUrl(s, spec)) &&
       !baseSpecs.find(s => JSON.stringify(s) === JSON.stringify(spec))),
-    deleted: baseSpecs.filter(spec => !newSpecs.find(s => hasSameUrl(s, spec)))
+    delete: baseSpecs.filter(spec => !newSpecs.find(s => haveSameUrl(s, spec)))
   };
+  if (diff.add.length || diff.update.length || diff.delete.length) {
+    for (const type of ['add', 'update', 'delete']) {
+      for (const spec of diff[type]) {
+        log(`- ${type} ${getUrl(spec)}`);
+      }
+    }
+  }
+  else {
+    log(`- no diff found`);
+  }
   log(`Compute specs.json diff... done`);
 
-  log(`Build specs that were added...`);
-  diff.added = (diff.added.length === 0) ? [] :
-    await generateIndex(diff.added, {
-      previousIndex: baseIndex,
-      log: function(...msg) { log(' ', ...msg); } });
-  log(`Build specs that were added... done`);
+  log(`Delete specs that were dropped...`);
+  diff.delete = baseIndex.filter(spec => diff.delete.find(s => haveSameUrl(s, spec)));
+  let newIndex = baseIndex.filter(spec => !diff.delete.find(s => haveSameUrl(s, spec)));
+  log(`Delete specs that were dropped... done`);
 
-  log(`Build specs that were updated...`);
-  diff.updated = (diff.updated.length === 0) ? [] :
-    await generateIndex(diff.updated, {
-      previousIndex: baseIndex,
+  log(`Build new/updated entries...`);
+  const needBuild = []
+    .concat(diff.add)
+    .concat(diff.update)
+    .map(spec => [spec].concat(baseSpecs.filter(s => areDistinctSpecsInSameSeries(s, spec))))
+    .concat(diff.delete.map(spec => baseSpecs.filter(s => areDistinctSpecsInSameSeries(s, spec))))
+    .flat();
+  const built = (needBuild.length === 0) ? [] :
+    await generateIndex(needBuild, {
+      previousIndex: newIndex,
       log: function(...msg) { log(' ', ...msg); } });
-  log(`Build specs that were updated... done`);
-
-  log(`Retrieve specs that were dropped...`);
-  diff.deleted = diff.deleted.map(spec => baseIndex.find(s => hasSameUrl(s, spec)));
-  log(`Retrieve specs that were dropped... done`);
+  diff.add = diff.add.map(spec => built.find(s => haveSameUrl(s, spec)));
+  diff.update = diff.update.map(spec => built.find(s => haveSameUrl(s, spec)));
+  diff.seriesUpdate = built
+    .filter(spec =>
+      !diff.add.find(s => haveSameUrl(s, spec)) &&
+      !diff.update.find(s => haveSameUrl(s, spec)))
+    .filter(spec => !areIdentical(spec, baseIndex.find(s => s.url === spec.url)));
+  log(`Build new/updated entries... done`);
 
   if (diffType === "full") {
     log(`Create full new index...`);
-    const newIndex = baseIndex
-      .map(spec => {
-        const updated = diff.updated.find(s => hasSameUrl(s, spec));
-        return updated ?? spec;
-      })
-      .filter(spec => !diff.deleted.find(s => hasSameUrl(s, spec)));
-    diff.added.forEach(spec => newIndex.push(spec));
+    newIndex = newIndex
+      .filter(spec => !built.find(s => haveSameUrl(s, spec)))
+      .concat(built);
     newIndex.sort(compareSpecs);
     log(`Create full new index... done`);
     return newIndex;