diff --git a/scripts/migrate_csv_to_db.py b/scripts/migrate_csv_to_db.py
index bd1f3b1..921ef66 100644
--- a/scripts/migrate_csv_to_db.py
+++ b/scripts/migrate_csv_to_db.py
@@ -177,8 +177,14 @@ def load_csv_data(data_dir: Path) -> pd.DataFrame:
 def migrate_data(df: pd.DataFrame, geocode: bool = False):
     """Migrate DataFrame data to database."""
     
-    # Group by URN to get unique schools
-    school_data = df.groupby('urn').first().reset_index()
+    # Clean URN column - convert to integer, drop invalid values
+    df = df.copy()
+    df['urn'] = pd.to_numeric(df['urn'], errors='coerce')
+    df = df.dropna(subset=['urn'])
+    df['urn'] = df['urn'].astype(int)
+    
+    # Group by URN to get unique schools (use latest year's data)
+    school_data = df.sort_values('year', ascending=False).groupby('urn').first().reset_index()
     print(f"\nMigrating {len(school_data)} unique schools...")
     
     # Geocode if requested
@@ -208,6 +214,10 @@ def migrate_data(df: pd.DataFrame, geocode: bool = False):
             if not urn:
                 continue
             
+            # Skip if we've already added this URN (handles duplicates in source data)
+            if urn in urn_to_school_id:
+                continue
+            
             # Get geocoding data
             postcode = row.get('postcode')
             lat, lon = None, None