diff --git a/app/routes/import_export.py b/app/routes/import_export.py
index 870bd97..e3880d0 100644
--- a/app/routes/import_export.py
+++ b/app/routes/import_export.py
@@ -591,7 +591,12 @@ def import_csv_clients_route():
     Import clients from CSV file
 
     Expected multipart/form-data with 'file' field
-    Optional query parameter: skip_duplicates (default: true)
+    Optional query parameters:
+        - skip_duplicates (default: true): Whether to skip duplicate clients
+        - duplicate_detection_fields: Comma-separated list of fields to use for duplicate detection.
+          Can include 'name' for client name, or custom field names (e.g., 'debtor_number').
+          Examples: "debtor_number", "name,debtor_number", "erp_id"
+          If not provided, defaults to checking by name and all custom fields found in CSV.
     """
     try:
         if "file" not in request.files:
@@ -606,6 +611,13 @@ def import_csv_clients_route():
             return jsonify({"error": "File must be a CSV"}), 400
 
         skip_duplicates = request.args.get("skip_duplicates", "true").lower() == "true"
+        
+        # Parse duplicate detection fields
+        duplicate_detection_fields = None
+        duplicate_fields_param = request.args.get("duplicate_detection_fields", "").strip()
+        if duplicate_fields_param:
+            # Support comma-separated list: "debtor_number,erp_id" or "name,debtor_number"
+            duplicate_detection_fields = [field.strip() for field in duplicate_fields_param.split(",") if field.strip()]
 
         # Read file content
         file_bytes = file.read()
@@ -637,7 +649,11 @@ def import_csv_clients_route():
         # Perform import
         try:
             summary = import_csv_clients(
-                user_id=current_user.id, csv_content=csv_content, import_record=import_record, skip_duplicates=skip_duplicates
+                user_id=current_user.id, 
+                csv_content=csv_content, 
+                import_record=import_record, 
+                skip_duplicates=skip_duplicates,
+                duplicate_detection_fields=duplicate_detection_fields
             )
             response = jsonify({"success": True, "import_id": import_record.id, "summary": summary})
             response.headers["Content-Type"] = "application/json"
diff --git a/app/templates/import_export/index.html b/app/templates/import_export/index.html
index acb667f..46bc15b 100644
--- a/app/templates/import_export/index.html
+++ b/app/templates/import_export/index.html
@@ -57,11 +57,23 @@
                             <i class="fas fa-download mr-1"></i>{{ _('Download Template') }}
                         </a>
                     </div>
-                    <div class="mt-2">
+                    <div class="mt-2 space-y-2">
                         <label class="flex items-center space-x-2 text-sm text-gray-600 dark:text-gray-400">
                             <input type="checkbox" id="skip-duplicates" checked class="rounded">
-                            <span>{{ _('Skip duplicate clients (by name or custom field)') }}</span>
+                            <span>{{ _('Skip duplicate clients') }}</span>
                         </label>
+                        <div id="duplicate-detection-fields" class="ml-6 space-y-2 text-sm text-gray-600 dark:text-gray-400">
+                            <div class="font-medium">{{ _('Use these fields for duplicate detection:') }}</div>
+                            <label class="flex items-center space-x-2">
+                                <input type="checkbox" id="dup-field-name" class="rounded dup-field-checkbox">
+                                <span>{{ _('Client Name') }}</span>
+                            </label>
+                            <div class="flex items-center space-x-2">
+                                <label for="dup-custom-fields" class="text-sm">{{ _('Custom fields (comma-separated):') }}</label>
+                                <input type="text" id="dup-custom-fields" placeholder="{{ _('e.g., debtor_number, erp_id') }}" class="flex-1 px-2 py-1 text-sm border border-gray-300 dark:border-gray-600 rounded-md dark:bg-gray-700 dark:text-white">
+                            </div>
+                            <div class="text-xs text-gray-500 dark:text-gray-500">{{ _('Example: Enter "debtor_number" to detect duplicates by debtor number only (not by name)') }}</div>
+                        </div>
                     </div>
                     <div id="csv-clients-upload-status" class="mt-2 text-sm"></div>
                 </div>
@@ -328,8 +340,23 @@ async function handleCsvClientsUpload(input) {
         formData.append('csrf_token', csrfToken);
     }
     
+    // Collect duplicate detection fields
+    const duplicateFields = [];
+    if (document.getElementById('dup-field-name').checked) {
+        duplicateFields.push('name');
+    }
+    const customFieldsInput = document.getElementById('dup-custom-fields').value.trim();
+    if (customFieldsInput) {
+        // Split by comma and add each field
+        const customFields = customFieldsInput.split(',').map(f => f.trim()).filter(f => f);
+        duplicateFields.push(...customFields);
+    }
+    
     try {
-        const url = `/api/import/csv/clients?skip_duplicates=${skipDuplicates}`;
+        let url = `/api/import/csv/clients?skip_duplicates=${skipDuplicates}`;
+        if (duplicateFields.length > 0) {
+            url += `&duplicate_detection_fields=${encodeURIComponent(duplicateFields.join(','))}`;
+        }
         const response = await fetch(url, {
             method: 'POST',
             body: formData
@@ -647,6 +674,25 @@ function showFilteredExportForm() {
 document.addEventListener('DOMContentLoaded', function() {
     loadImportHistory();
     loadExportHistory();
+    
+    // Toggle duplicate detection fields visibility based on skip-duplicates checkbox
+    const skipDuplicatesCheckbox = document.getElementById('skip-duplicates');
+    const duplicateDetectionFields = document.getElementById('duplicate-detection-fields');
+    
+    function toggleDuplicateFields() {
+        if (skipDuplicatesCheckbox && duplicateDetectionFields) {
+            if (skipDuplicatesCheckbox.checked) {
+                duplicateDetectionFields.style.display = 'block';
+            } else {
+                duplicateDetectionFields.style.display = 'none';
+            }
+        }
+    }
+    
+    if (skipDuplicatesCheckbox && duplicateDetectionFields) {
+        skipDuplicatesCheckbox.addEventListener('change', toggleDuplicateFields);
+        toggleDuplicateFields(); // Initialize on page load
+    }
 });
 </script>
 {% endblock %}
diff --git a/app/utils/data_import.py b/app/utils/data_import.py
index e7a876e..6e97f72 100644
--- a/app/utils/data_import.py
+++ b/app/utils/data_import.py
@@ -560,7 +560,7 @@ def _parse_datetime(datetime_str):
     return None
 
 
-def import_csv_clients(user_id, csv_content, import_record, skip_duplicates=True):
+def import_csv_clients(user_id, csv_content, import_record, skip_duplicates=True, duplicate_detection_fields=None):
     """
     Import clients from CSV file
 
@@ -579,7 +579,11 @@ def import_csv_clients(user_id, csv_content, import_record, skip_duplicates=True
         user_id: ID of the user importing data
         csv_content: String content of CSV file
         import_record: DataImport model instance to track progress
-        skip_duplicates: If True, skip clients that already exist (by name or custom field match)
+        skip_duplicates: If True, skip clients that already exist
+        duplicate_detection_fields: List of field names to use for duplicate detection.
+            Can include 'name' for client name, or custom field names (e.g., 'debtor_number').
+            If None, defaults to ['name'] plus all custom fields found in the CSV.
+            Examples: ['debtor_number'], ['name', 'debtor_number'], ['erp_id']
 
     Returns:
         Dictionary with import statistics
@@ -617,24 +621,42 @@ def import_csv_clients(user_id, csv_content, import_record, skip_duplicates=True
 
             # Check for duplicates if skip_duplicates is True
             if skip_duplicates:
-                existing_client = Client.query.filter_by(name=client_name).first()
+                existing_client = None
                 
-                # Also check by custom fields if provided (e.g., ERP ID)
-                if not existing_client:
-                    # Look for common custom field keys that might indicate duplicates
+                # Determine which fields to use for duplicate detection
+                if duplicate_detection_fields is not None:
+                    # Use explicitly specified fields
+                    detection_fields = duplicate_detection_fields
+                else:
+                    # Default: check by name + all custom fields found in CSV
+                    detection_fields = ['name']
+                    # Add all custom fields found in CSV
                     for key in row.keys():
                         if key.startswith("custom_field_"):
                             field_name = key.replace("custom_field_", "")
-                            field_value = row.get(key, "").strip()
-                            if field_value:
-                                # Check if any client has this custom field value
-                                all_clients = Client.query.all()
-                                for client in all_clients:
-                                    if client.custom_fields and client.custom_fields.get(field_name) == field_value:
-                                        existing_client = client
-                                        break
-                                if existing_client:
+                            if field_name not in detection_fields:
+                                detection_fields.append(field_name)
+                
+                # Check each specified field for duplicates
+                for field in detection_fields:
+                    if field == 'name':
+                        # Check by client name
+                        existing_client = Client.query.filter_by(name=client_name).first()
+                        if existing_client:
+                            break
+                    else:
+                        # Check by custom field
+                        csv_key = f"custom_field_{field}"
+                        field_value = row.get(csv_key, "").strip()
+                        if field_value:
+                            # Check if any client has this custom field value
+                            all_clients = Client.query.all()
+                            for client in all_clients:
+                                if client.custom_fields and client.custom_fields.get(field) == field_value:
+                                    existing_client = client
                                     break
+                            if existing_client:
+                                break
                 
                 if existing_client:
                     skipped += 1