diff --git a/integrations/acquisition/covidcast_nowcast/test_csv_uploading.py b/integrations/acquisition/covidcast_nowcast/test_csv_uploading.py
index b401b3ae0..b92ed5065 100644
--- a/integrations/acquisition/covidcast_nowcast/test_csv_uploading.py
+++ b/integrations/acquisition/covidcast_nowcast/test_csv_uploading.py
@@ -72,8 +72,6 @@ def test_uploading(self):
     success_dir = '/common/covidcast_nowcast/archive/successful/src/'
     failed_dir = '/common/covidcast_nowcast/archive/failed/src/'
     os.makedirs(receiving_dir, exist_ok=True)
-    os.makedirs(success_dir, exist_ok=True)
-    os.makedirs(failed_dir, exist_ok=True)
 
     # valid
     with open(receiving_dir + '20200419_state_sig.csv', 'w') as f:
@@ -118,3 +116,39 @@ def test_uploading(self):
       }],
       'message': 'success',
     })
+
+  @patch('delphi.epidata.acquisition.covidcast_nowcast.load_sensors.CsvImporter.find_csv_files',
+         new=FIXED_ISSUE_IMPORTER)
+  def test_duplicate_row(self):
+    """Test duplicate unique keys are updated."""
+
+    # print full diff if something unexpected comes out
+    self.maxDiff=None
+
+    receiving_dir = '/common/covidcast_nowcast/receiving/src/'
+    os.makedirs(receiving_dir, exist_ok=True)
+
+    with open(receiving_dir + '20200419_state_sig.csv', 'w') as f:
+      f.write('sensor_name,geo_value,value\n')
+      f.write('testsensor,ca,1\n')
+    main()
+    with open(receiving_dir + '20200419_state_sig.csv', 'w') as f:
+      f.write('sensor_name,geo_value,value\n')
+      f.write('testsensor,ca,2\n')
+    main()
+
+    # most most recent value is the one stored
+    response = Epidata.covidcast_nowcast(
+      'src', 'sig', 'testsensor', 'day', 'state', 20200419, 'ca')
+    self.assertEqual(response, {
+      'result': 1,
+      'epidata': [{
+        'time_value': 20200419,
+        'geo_value': 'ca',
+        'value': 2,
+        'issue': 20200421,
+        'lag': 2,
+        'signal': 'sig',
+      }],
+      'message': 'success',
+    })
diff --git a/src/acquisition/covidcast_nowcast/load_sensors.py b/src/acquisition/covidcast_nowcast/load_sensors.py
index 2c633d183..16c4ef2b0 100644
--- a/src/acquisition/covidcast_nowcast/load_sensors.py
+++ b/src/acquisition/covidcast_nowcast/load_sensors.py
@@ -1,4 +1,5 @@
 from shutil import move
+import os
 import time
 
 import delphi.operations.secrets as secrets
@@ -7,6 +8,8 @@
 from delphi.epidata.acquisition.covidcast.csv_importer import CsvImporter
 
 SENSOR_CSV_PATH = "/common/covidcast_nowcast/receiving/"
+SUCCESS_DIR = "archive/successful"
+FAIL_DIR = "archive/failed"
 TABLE_NAME = "covidcast_nowcast"
 DB_NAME = "epidata"
 CSV_DTYPES = {"sensor_name": str, "geo_value": str, "value": float}
@@ -33,17 +36,20 @@ def main(csv_path: str = SENSOR_CSV_PATH) -> None:
     """
     user, pw = secrets.db.epi
     engine = sqlalchemy.create_engine(f"mysql+pymysql://{user}:{pw}@{secrets.db.host}/{DB_NAME}")
-    for filepath, attributes in CsvImporter.find_csv_files(csv_path):
-        if attributes is None:
-            move(filepath, filepath.replace("receiving", "archive/failed"))
+    for filepath, attribute in CsvImporter.find_csv_files(csv_path):
+        if attribute is None:
+            _move_after_processing(filepath, success=False)
             continue
         try:
-            data = load_and_prepare_file(filepath, attributes)
-            data.to_sql(TABLE_NAME, engine, if_exists="append", index=False)
+            data = load_and_prepare_file(filepath, attribute)
+            conn = engine.connect()
+            with conn.begin():
+                method = _create_upsert_method(sqlalchemy.MetaData(conn))
+                data.to_sql(TABLE_NAME, engine, if_exists="append", method=method, index=False)
         except Exception:
-            move(filepath, filepath.replace("receiving", "archive/failed"))
+            _move_after_processing(filepath, success=False)
             raise
-        move(filepath, filepath.replace("receiving", "archive/successful"))
+        _move_after_processing(filepath, success=True)
 
 
 def load_and_prepare_file(filepath: str, attributes: tuple) -> pd.DataFrame:
@@ -75,5 +81,23 @@ def load_and_prepare_file(filepath: str, attributes: tuple) -> pd.DataFrame:
     return data
 
 
+def _move_after_processing(filepath, success):
+    archive_dir = SUCCESS_DIR if success else FAIL_DIR
+    new_dir = os.path.dirname(filepath).replace(
+        "receiving", archive_dir)
+    os.makedirs(new_dir, exist_ok=True)
+    move(filepath, filepath.replace("receiving", archive_dir))
+    print(f"{filepath} moved to {archive_dir}")
+
+
+def _create_upsert_method(meta):
+    def method(table, conn, keys, data_iter):
+        sql_table = sqlalchemy.Table(table.name, meta, autoload=True)
+        insert_stmt = sqlalchemy.dialects.mysql.insert(sql_table).values([dict(zip(keys, data)) for data in data_iter])
+        upsert_stmt = insert_stmt.on_duplicate_key_update({x.name: x for x in insert_stmt.inserted})
+        conn.execute(upsert_stmt)
+    return method
+
+
 if __name__ == "__main__":
     main()