Update duplicate handling docs to include pension tables

- Document composite primary key handling for pension tables
- Add code examples for both single and composite key duplicate detection
- List all pension-related tables with duplicate protection

docs/DUPLICATE_HANDLING.md

@@ -22,16 +22,30 @@ The import system now implements multiple layers of duplicate protection:
 
 ### 1. In-Memory Duplicate Tracking
 ```python
+# For single primary key
 seen_in_import = set()
+
+# For composite primary key (e.g., file_no + version)
+seen_in_import = set()  # stores tuples like (file_no, version)
+composite_key = (file_no, version)
 ```
-Tracks IDs encountered during the current import session. If an ID is seen twice in the same file, only the first occurrence is imported.
+Tracks IDs or composite key combinations encountered during the current import session. If a key is seen twice in the same file, only the first occurrence is imported.
 
 ### 2. Database Existence Check
 Before importing each record, checks if it already exists:
 ```python
+# For single primary key (e.g., rolodex)
 if db.query(Rolodex).filter(Rolodex.id == rolodex_id).first():
     result['skipped'] += 1
     continue
+
+# For composite primary key (e.g., pensions with file_no + version)
+if db.query(Pensions).filter(
+    Pensions.file_no == file_no,
+    Pensions.version == version
+).first():
+    result['skipped'] += 1
+    continue
 ```
 
 ### 3. Graceful Batch Failure Handling
@@ -74,6 +88,10 @@ This is **not an error** - it means the system is protecting data integrity.
 Currently implemented for:
 - ✅ `rolodex` (primary key: id)
 - ✅ `filetype` (primary key: file_type)
+- ✅ `pensions` (composite primary key: file_no, version)
+- ✅ `pension_death` (composite primary key: file_no, version)
+- ✅ `pension_separate` (composite primary key: file_no, version)
+- ✅ `pension_results` (composite primary key: file_no, version)
 
 Other tables should be updated if they encounter similar issues.