diff --git a/dicom-viewer/src/App.tsx b/dicom-viewer/src/App.tsx
index 25fe084..3fedfbf 100644
--- a/dicom-viewer/src/App.tsx
+++ b/dicom-viewer/src/App.tsx
@@ -2459,12 +2459,23 @@ function App({ container_id, imageStacks, autoCacheStack, annotationJson, viewer
     const seriesGroups = await deriveSeriesGroupsFromImageIds(imageIds);
     const primarySeries = seriesGroups[0]?.imageIds || imageIds;
 
-    // Add the incoming stack to the panel and capture its index.
-    let newStackIdx = 0;
+
+    // Check if a stack with the same imageIds already exists
+    let existingStackIdx = -1;
+    let newStackIdx = -1;
     setAvailableStacks(prev => {
+      existingStackIdx = prev.findIndex(
+        s =>
+          s.imageIds.length === imageIds.length &&
+          s.imageIds.every((id, idx) => id === imageIds[idx])
+      );
+      if (existingStackIdx !== -1) {
+        return prev;
+      }
       newStackIdx = prev.length;
       return [...prev, { imageIds, name, seriesGroups }];
     });
+    const stackIdxToUse = existingStackIdx !== -1 ? existingStackIdx : newStackIdx;
 
     const resolvedZone = resolveDropZone(requestedZone);
     if (resolvedZone === 'center') {
@@ -2472,7 +2483,7 @@ function App({ container_id, imageStacks, autoCacheStack, annotationJson, viewer
       try {
         setViewportModes(prev => { const n = [...prev]; n[viewportIdx] = 'stack'; return n; });
         setViewportImageIds(prev => { const n = [...prev]; n[viewportIdx] = primarySeries; return n; });
-        setSelectedStackIdx(prev => { const n = [...prev]; n[viewportIdx] = newStackIdx; return n; });
+        setSelectedStackIdx(prev => { const n = [...prev]; n[viewportIdx] = stackIdxToUse; return n; });
         setSeriesGroupsByViewport(prev => {
           const n = [...prev];
           n[viewportIdx] = seriesGroups;
@@ -2504,7 +2515,7 @@ function App({ container_id, imageStacks, autoCacheStack, annotationJson, viewer
       try {
         setViewportModes(prev => { const n = [...prev]; n[viewportIdx] = 'stack'; return n; });
         setViewportImageIds(prev => { const n = [...prev]; n[viewportIdx] = imageIds; return n; });
-        setSelectedStackIdx(prev => { const n = [...prev]; n[viewportIdx] = newStackIdx; return n; });
+        setSelectedStackIdx(prev => { const n = [...prev]; n[viewportIdx] = stackIdxToUse; return n; });
         setActiveViewport(viewportIdx);
         await setLoadedImageIdsAndCacheMeta(imageIds);
       } finally {
@@ -2518,6 +2529,7 @@ function App({ container_id, imageStacks, autoCacheStack, annotationJson, viewer
     const insertedViewportIndex = targetNewRow * newCols + targetNewCol;
 
     const nextImageIds = Array(MAX_VIEWPORTS).fill([] as string[]);
+    // If you use newStackIdx below, ensure it is only used if a new stack was added
     const nextModes = Array(MAX_VIEWPORTS).fill('stack') as Array<'stack' | 'volume'>;
     const nextSelected = Array(MAX_VIEWPORTS).fill(0);
     const nextSeriesGroups = Array(MAX_VIEWPORTS).fill(null).map(() => [] as StackSeriesGroup[]);