diff --git a/asreview/data/base.py b/asreview/data/base.py
index fa6e9557d..ea930d50a 100644
--- a/asreview/data/base.py
+++ b/asreview/data/base.py
@@ -510,7 +510,7 @@ def duplicated(self, pid="doi"):
             if is_string_dtype(self.df[pid]) or is_object_dtype(self.df[pid]):
                 s_pid = self.df[pid].str.strip().replace("", None)
                 if pid == "doi":
-                    s_pid = s_pid.str.replace(
+                    s_pid = s_pid.str.lower().str.replace(
                         r"^https?://(www\.)?doi\.org/", "", regex=True
                     )
             else:
diff --git a/tests/test_data.py b/tests/test_data.py
index 649460f6e..7e0504585 100644
--- a/tests/test_data.py
+++ b/tests/test_data.py
@@ -143,16 +143,17 @@ def test_duplicated():
         'doi': ['https://www.doi.org/10.1000/xyz',
                 'https://www.doi.org/10.1000/abc',
                 'https://www.doi.org/10.1000/xyz',
+                'https://www.doi.org/10.1000/XYZ',
                 '10.1000/xyz',
                 '10.1000/xyz',
                 'http://www.doi.org/10.1000/xyz',
                 'https://doi.org/10.1000/xyz'],
-        'title': ['T1', 'T2', 'T3', 'T4', 'T1', 'T2', 'T3'],
-        'abstract': ['A1', 'A2', 'A3', 'A4', 'A1', 'A2', 'A3']
+        'title': ['T1', 'T2', 'T3', 'T3', 'T4', 'T1', 'T2', 'T3'],
+        'abstract': ['A1', 'A2', 'A3', 'A3', 'A4', 'A1', 'A2', 'A3']
     }))
 
     # Call the function and get the result
     result = instance.duplicated()
 
     # Check the result
-    assert result.equals(pd.Series([False, False, True, True, True, True, True]))
+    assert result.equals(pd.Series([False, False, True, True, True, True, True, True]))