Add note about Post/Redirect/Get pattern in by_example tutorial

nbs/tutorials/by_example.ipynb

@@ -680,6 +680,14 @@
     "We re-render the entire homepage to show the newly added message. This is fine, but modern web apps often don't re-render the entire page, they just update a part of the page. In fact even very complicated applications are often implemented as 'Single Page Apps' (SPAs). This is where HTMX comes in."
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "prg-note",
+   "metadata": {},
+   "source": [
+    "> **Note:** With traditional form submissions, refreshing the page after a POST will prompt \\\"Resubmit form data?\\\" — potentially causing duplicate entries. The standard fix is `return Redirect('/')` instead of `return home()`, known as the [Post/Redirect/Get](https://en.wikipedia.org/wiki/Post/Redirect/Get) pattern. With HTMX (covered next), this isn't needed since submissions happen via AJAX."
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "0e7066db",