diff --git a/public/js/modals/invoice-modal.js b/public/js/modals/invoice-modal.js
index 7afd681..7a38b55 100644
--- a/public/js/modals/invoice-modal.js
+++ b/public/js/modals/invoice-modal.js
@@ -98,9 +98,20 @@ async function loadInvoiceForEdit(invoiceId) {
     const recurringInterval = document.getElementById('invoice-recurring-interval');
     const recurringGroup = document.getElementById('invoice-recurring-group');
     if (recurringCb) {
-        recurringCb.checked = data.invoice.is_recurring || false;
-        if (recurringInterval) recurringInterval.value = data.invoice.recurring_interval || 'monthly';
-        if (recurringGroup) recurringGroup.style.display = data.invoice.is_recurring ? 'block' : 'none';
+        const isGeneratedRecurringChild = !!data.invoice.recurring_source_id;
+        const canBeRecurringMaster = !isGeneratedRecurringChild;
+
+        recurringCb.checked = canBeRecurringMaster && (data.invoice.is_recurring || false);
+        recurringCb.disabled = !canBeRecurringMaster;
+
+        if (recurringInterval) {
+            recurringInterval.value = data.invoice.recurring_interval || 'monthly';
+            recurringInterval.disabled = !canBeRecurringMaster;
+        }
+
+        if (recurringGroup) {
+            recurringGroup.style.display = recurringCb.checked ? 'block' : 'none';
+        }
     }
 
     // Load items
diff --git a/src/routes/invoices.js b/src/routes/invoices.js
index a746dbd..7207e4e 100644
--- a/src/routes/invoices.js
+++ b/src/routes/invoices.js
@@ -269,11 +269,46 @@ router.put('/:id', async (req, res) => {
                 [customer_id, invoice_date, terms, auth_code, tax_exempt, tax_rate, subtotal, tax_amount, total, scheduled_send_date || null, bill_to_name || null, id]
             );
         }
-        const next_recurring_date = is_recurring ? calculateNextRecurringDate(invoice_date, recurring_interval) : null;
-        await client.query(
-            'UPDATE invoices SET is_recurring = $1, recurring_interval = $2, next_recurring_date = $3 WHERE id = $4',
-            [is_recurring || false, recurring_interval || null, next_recurring_date, id]
+        
+        // Preserve existing next_recurring_date when editing an already-recurring invoice.
+        // Otherwise editing an old invoice can move next_recurring_date backwards and create duplicates.
+        const existingRecurringResult = await client.query(
+            'SELECT is_recurring, next_recurring_date, recurring_source_id FROM invoices WHERE id = $1',
+            [id]
         );
+
+        const existingRecurring = existingRecurringResult.rows[0];
+
+        let next_recurring_date = null;
+
+        // Automatically generated child invoices should not become recurring masters.
+        // recurring_source_id != null means: this invoice was created from another recurring invoice.
+        const isGeneratedRecurringChild = !!existingRecurring?.recurring_source_id;
+
+        if (isGeneratedRecurringChild) {
+            next_recurring_date = null;
+        } else if (is_recurring) {
+            next_recurring_date = existingRecurring?.next_recurring_date
+                ? existingRecurring.next_recurring_date
+                : calculateNextRecurringDate(invoice_date, recurring_interval);
+        }
+
+        await client.query(
+            `
+            UPDATE invoices
+            SET is_recurring = $1,
+                recurring_interval = $2,
+                next_recurring_date = $3
+            WHERE id = $4
+            `,
+            [
+                isGeneratedRecurringChild ? false : (is_recurring || false),
+                isGeneratedRecurringChild ? null : (recurring_interval || null),
+                next_recurring_date,
+                id
+            ]
+        );
+
         // Delete and re-insert items
         await client.query('DELETE FROM invoice_items WHERE invoice_id = $1', [id]);
         for (let i = 0; i < items.length; i++) {