diff --git a/src/hest/readers.py b/src/hest/readers.py
index 6d9b1cb..0158284 100644
--- a/src/hest/readers.py
+++ b/src/hest/readers.py
@@ -1241,6 +1241,7 @@ def pool_bins_visiumhd(adata: sc.AnnData, pixel_size: float, dst_bin_size_um=128
         sc.AnnData: AnnData object, each row in .obs represents a bin, each row in `.X` represents the sum of visium-hd sub-bins (of size `src_bin_size_um`) within that larger bin (of size `dst_bin_size_um`). Center coordinates of each bin (in pixel on WSI) are in adata.obsm['spatial']
     """
     import scanpy as sc
+    from scipy.sparse import issparse
 
     if src_bin_size_um >= dst_bin_size_um:
         raise ValueError("dst_bin_size_um needs to be larger than src_bin_size_um")
@@ -1278,7 +1279,11 @@ def pool_bins_visiumhd(adata: sc.AnnData, pixel_size: float, dst_bin_size_um=128
 
     for i in range(nb_chunks):
         start, end = i * chunk_len, min((i + 1) * chunk_len, len(c))
-        spot_grid_np[c[start:end]] += adata.X[start:end]
+        counts = adata.X[start:end]
+        if issparse(counts):
+            counts = counts.toarray()
+        # Repeated destination indices must accumulate every source bin.
+        np.add.at(spot_grid_np, c[start:end], counts)
 
     
     expression_df = pd.DataFrame(spot_grid_np, columns=spot_grid.columns)
@@ -1460,4 +1465,4 @@ def process_raw_samples(
             patching_kwargs=patching_kwargs
         )
 
-        save_meta(dict(row))
\ No newline at end of file
+        save_meta(dict(row))
diff --git a/tests/hest_tests.py b/tests/hest_tests.py
index 73710c8..a001881 100644
--- a/tests/hest_tests.py
+++ b/tests/hest_tests.py
@@ -19,7 +19,7 @@
     raise ImportError(f"Importing 'hest' took too long ({elapsed_time:.2f} seconds). Maximum allowed time is {MAX_HEST_IMPORT_S} seconds. Please, keep large large imports conditional")
 
 from hest.autoalign import autoalign_visium
-from hest.readers import VisiumReader
+from hest.readers import VisiumReader, pool_bins_visiumhd
 from hest.HESTData import ensembl_id_to_gene
 from hest.utils import load_image
 
@@ -37,6 +37,59 @@ def download_hest(patterns, local_dir):
                 zip_ref.extractall(seg_dir)
 
 
+class TestVisiumHDPooling(unittest.TestCase):
+    def test_count_conservation(self):
+        from itertools import product
+        from tempfile import TemporaryDirectory
+
+        import anndata as ad
+        import numpy as np
+        import pandas as pd
+        from scipy.sparse import csr_matrix, csc_matrix, issparse
+
+        # Each destination has repeated indices and a final zero-count source bin.
+        counts = np.array([
+            [1, 10, 0], [2, 20, 0], [3, 30, 0], [0, 0, 0],
+            [4, 40, 0], [5, 50, 0], [6, 60, 0], [0, 0, 0],
+        ], dtype=np.float32)
+        obs = pd.DataFrame({
+            'pxl_col_in_fullres': [8, 24, 8, 24, 136, 152, 136, 152],
+            'pxl_row_in_fullres': [8, 8, 24, 24, 8, 8, 24, 24],
+        }, index=[str(i) for i in range(8)])
+        genes = ['gene_b', 'gene_a', 'zero_gene']
+        orders = [np.arange(8), np.arange(8)[::-1], np.array([0, 4, 1, 5, 2, 6, 3, 7])]
+
+        with TemporaryDirectory() as tmp:
+            for storage, backed, chunk_len, order in product(
+                (np.array, csr_matrix, csc_matrix), (False, True), (1, 2, 3, 50000), orders
+            ):
+                with self.subTest(storage=storage.__name__, backed=backed,
+                                  chunk_len=chunk_len, order=order.tolist()):
+                    adata = ad.AnnData(
+                        storage(counts[order]), obs=obs.iloc[order].copy(),
+                        var=pd.DataFrame(index=genes),
+                    )
+                    if backed:
+                        path = os.path.join(tmp, 'counts.h5ad')
+                        adata.write_h5ad(path)
+                        adata = ad.read_h5ad(path, backed='r')
+                    try:
+                        pooled = pool_bins_visiumhd(adata, pixel_size=1.0, chunk_len=chunk_len)
+                        np.testing.assert_array_equal(pooled.X, [[6, 60, 0], [15, 150, 0]])
+                        self.assertEqual(pooled.X.sum(), counts.sum())
+                        self.assertEqual(pooled.var_names.tolist(), genes)
+                        np.testing.assert_array_equal(pooled.obsm['spatial'], [[72, 72], [200, 72]])
+                        np.testing.assert_array_equal(pooled.obs['array_col'], [0, 1])
+                        np.testing.assert_array_equal(pooled.obs['array_row'], [0, 0])
+                        remaining = adata.X[:]
+                        if issparse(remaining):
+                            remaining = remaining.toarray()
+                        np.testing.assert_array_equal(remaining, counts[order])
+                    finally:
+                        if backed:
+                            adata.file.close()
+
+
 class TestHESTReader(unittest.TestCase):
     
     @classmethod
@@ -227,6 +280,7 @@ def test_saving(self):
     
     loader = unittest.TestLoader()
     suite = loader.loadTestsFromTestCase(TestHESTData)
+    suite.addTests(loader.loadTestsFromTestCase(TestVisiumHDPooling))
     # suite = unittest.TestSuite()
     #suite.addTest(TestHESTData('test_spatialdata'))
     result = unittest.TextTestRunner(verbosity=2).run(suite)