diff --git a/static/js/app.js b/static/js/app.js
index cf513db..21376a4 100644
--- a/static/js/app.js
+++ b/static/js/app.js
@@ -4505,7 +4505,30 @@ function exitReaderImmersiveMode() {
   document.removeEventListener('click', _immHandleTap);
 }
 
+// Serializes openBook: it empties #reader-content, then appends the chapters
+// across several awaits. A second call in between (a second tap while a slow
+// e-reader is still decrypting) appended the whole book a second time — and
+// progress saved from that doubled DOM had block indices beyond the real book,
+// which then out-ranked every real position on the server.
+let _openBookInFlight = null;
+let _repairProgressAnchor = false; // stored anchor didn't fit this book
+
 async function openBook(bookId) {
+  if (_openBookInFlight) {
+    if (_openBookInFlight.bookId === bookId) return _openBookInFlight.promise;
+    await _openBookInFlight.promise.catch(() => {});
+  }
+  const promise = _openBookOnce(bookId);
+  _openBookInFlight = {bookId, promise};
+  try {
+    return await promise;
+  } finally {
+    if (_openBookInFlight && _openBookInFlight.promise === promise) _openBookInFlight = null;
+  }
+}
+
+async function _openBookOnce(bookId) {
+  _repairProgressAnchor = false;
   const overlay = $('reader-overlay');
   const contentEl = $('reader-content');
   const titleEl = $('reader-title');
@@ -4690,7 +4713,14 @@ async function openBook(bookId) {
         }
         await new Promise(r => requestAnimationFrame(r));
         if (!restoreFromAnchor(contentEl, anchor) && fraction > 0) {
-          contentEl.scrollTop = fraction * (contentEl.scrollHeight - contentEl.clientHeight);
+          restoreFromAnchor(contentEl, _anchorForFraction(contentEl, fraction));
+        }
+        // An anchor past the end of this book can only come from a broken
+        // render. The server keeps the furthest block index, so it would block
+        // every real position from now on: overwrite it with the next save.
+        const [storedBlock] = _anchorParts(anchor);
+        if (storedBlock >= contentEl.querySelectorAll(EPUB_BLOCK_SELECTOR).length) {
+          _repairProgressAnchor = true;
         }
       }
     } catch (e) {}
@@ -4844,6 +4874,13 @@ async function saveReaderProgress(force = false) {
     fraction = contentEl.scrollTop / (contentEl.scrollHeight - contentEl.clientHeight || 1);
   }
 
+  // Chapter wrappers carry unique ids; seeing one twice means the book was
+  // rendered twice. Positions from that DOM are wrong — don't store them.
+  if (!currentPdfDoc && contentEl.querySelectorAll('#epub-chapter-0').length > 1) {
+    console.warn('Book rendered twice, progress not saved');
+    return;
+  }
+
   let anchor = '';
   if (!currentPdfDoc) {
     anchor = getPositionAnchor(contentEl);
@@ -4854,9 +4891,11 @@ async function saveReaderProgress(force = false) {
   }
   fraction = Math.min(1.0, Math.max(0.0, fraction));
 
-  _updateCachedBookProgress(currentBookId, fraction, anchor, force);
+  _updateCachedBookProgress(currentBookId, fraction, anchor, force || (_repairProgressAnchor && !!anchor));
 
-  const body = JSON.stringify({scroll_fraction: fraction, position_anchor: anchor, ...(force && {force: true})});
+  const repair = _repairProgressAnchor && !!anchor;
+  if (repair) _repairProgressAnchor = false;
+  const body = JSON.stringify({scroll_fraction: fraction, position_anchor: anchor, ...((force || repair) && {force: true})});
   _lastProgressBeacon = {url: `/books/${currentBookId}/progress/`, body};
 
   try {
diff --git a/static/js/sw.js b/static/js/sw.js
index 2b72b57..c36b133 100644
--- a/static/js/sw.js
+++ b/static/js/sw.js
@@ -2,7 +2,7 @@
  * diora service worker — caches the app shell for offline use.
  */
 
-const CACHE = 'diora-v47';
+const CACHE = 'diora-v48';
 const PODCAST_CACHE = 'diora-podcast-v1';
 const SHELL = [
   '/static/css/app.css',