Turbo Drive re-evaluates ES modules on each navigation, duplicating side effects

Direct answer

Turbo Drive has no built-in option to detect that an ES module was already evaluated and skip it on the next visit. Whether you use full body replacement in Turbo 7 or morphing in Turbo 8, incoming <script type="module"> elements are treated as new script elements, so their top-level code can run again. For once-per-session initialization, export an idempotent init function and guard it with a module-level or <code>window</code> flag inside a <code>turbo:load</code> listener.

Confirmed behavior

• Turbo Drive navigates without reloading the document and retains the <head>.
• The body is replaced or, in Turbo 8, morphed. Scripts in the retained head persist; scripts in the body are subject to replacement or morphing.
• ES-module URL deduplication applies within a single document evaluation, not across Turbo visits that re-insert script elements.
• No public Turbo API or confirmed roadmap item exists for marking a module as already evaluated across visits.

Likely cause of duplicated side effects

Top-level code in a module--event listener registration, WebSocket creation, third-party widget setup--runs whenever Turbo inserts that module script into the document. The morph renderer diffs DOM nodes but does not preserve JavaScript execution state for a module URL that appears in both the cached and new page.

Recommended pattern

1. Move session-long setup out of top-level module code into an exported function.
2. Call that function from <code>turbo:load</code> only if a guard flag is unset.
3. Set the flag after the first successful run.
4. Clean up listeners, timers, and WebSockets on <code>turbo:before-visit</code> so repeated visits do not accumulate resources.

import { initWidget } from "./widget.js";

let initialized = false;

document.addEventListener("turbo:load", () => {
  if (initialized) return;
  initWidget();
  initialized = true;
});

document.addEventListener("turbo:before-visit", () => {
  // teardown here if initWidget created listeners or connections
});

Use <code>sessionStorage</code> instead of a memory flag if the guard must survive a full page reload in the same tab. Use a plain module flag if the guard should reset on reload.

Verification

1. Check <code>Turbo.VERSION</code> in the browser console to distinguish Turbo 7 replacement from Turbo 8 morphing.
2. Confirm whether the duplicating module scripts are external scripts in the retained head, inline modules, or scripts located in the replaced body.
3. Log module top-level execution and open WebSocket counts across two Turbo visits; the second visit should not repeat setup when the guard works.

Missing diagnostic: confirm the script placement (head versus body) and Turbo version; this determines whether head persistence or body morphing is the cause and whether a head-resident bootstrap script is a simpler fix.