Commerce begin_checkout event can fire multiple times with ajax panes

Patch attached.

Just found this as well. Patch partially helps, but a page refresh still counts twice, not sure if that's expected or not. There's also 🐛 Drupal Commerce purchase event gets sent multiple times if someone refreshes the checkout success page Active for a similar problem on purchase/checkout complete.

Wondering if google_tag should maybe have a built-in feature for unique events where it stores some kind of identifier in session/tempstore.

The SEO expert who flagged this issue for our site suggested it would be expected for begin_checkout to fire every time the first page of checkout is displayed, such as if someone navigates away from it and then comes back. If that's true, I think it would be tough to prevent it on page refresh, since that's going to be very similar. I guess we could look at the referrer, perhaps.

For the purchase event, the situation would be different. That one should only ever fire once per order. It should probably happen on the order placed or order paid event rather than looking at the checkout step.

I agree about the idea of a tempstore before firing, if thats even possible.. But there is likely some work needed to get this issue totally fixed per Berdir's comment.

For commerce event tracking we've been using https://www.drupal.org/project/commerce_google_tag_manager → (its latest release is hidden because it is Drupal 8, 9 only, plus the events are done for Universal analytics, not GA4) and there we never had an issue with events firing multiple times. I think there they also use tempstore. Maybe we could apply their solution here as well.

@japerry: I'm not entirely sure that the purchase and this event are exactly the same and need the same fix per #4. Purchase definitely should not fire more than once ever for the same order, but I can see use cases where this might be desired for this event, like when you abort and re-enter checkout.

But if you have a payment and then return on a failed payment to the first step then that's also a problem and likely a bigger one than not firing the event again.

commerce_google_tag_manager did not have a persistent key value store, it just used a request subscriber and only acted on GET requests, which is not unlike the current patch here.

FWIW, that said, we are using this patch now in production and did not yet get any complaints about it, so it's definitely an improvement over HEAD and one option would be to commit this and reopen 🐛 Drupal Commerce purchase event gets sent multiple times if someone refreshes the checkout success page Active to implement a persistent deduplication using tempstore or something.