diff --git a/parafac2/normalize.py b/parafac2/normalize.py
index 8908965..8ac5041 100644
--- a/parafac2/normalize.py
+++ b/parafac2/normalize.py
@@ -79,6 +79,9 @@ def get_deviance(data: csr_array) -> np.ndarray:
     where y=0 or n-y=0, and ensures the final deviance values are non-negative
     before taking the square root.
     """
+    # merge duplicate entries in the sparse matrix by summing their values
+    data.sum_duplicates()
+    data.eliminate_zeros()
 
     # counts per gene
     pi_j = data.sum(axis=0)
@@ -104,7 +107,7 @@ def get_deviance(data: csr_array) -> np.ndarray:
     # Term 1: y * log(y / mu) = xlogy(y, y) - xlogy(y, mu)
     # xlogy handles y=0 case correctly returning 0.
     row, col = data.nonzero()
-    mu_ij_nn = n_i_col[row, 0] * pi_j[0, col]
+    mu_ij_nn = n_i[row] * pi_j[col]
     term1 = data.data * np.log(data.data / mu_ij_nn)
 
     # Term 2: (n-y) * log((n-y) / (n-mu)) = xlogy(n-y, n-y) - xlogy(n-y, n-mu)