Bug 40150: Prevent uncaught error on multiple attempts to 'define' on 'CustomElementsRegistry' in islands.ts

If the name is already in use, we just return.

It might make sense to have a console.debug() or similar to notify
developers that this is happening. Open for other opinions.

To test:
1) Apply the first patch which adds an additional hydrate call in main-container.inc
2) Go to acqui/acqui-home.pl
3) Verify that you see this error in the dev tool's console tab:
Uncaught (in promise) NotSupportedError: Failed to execute 'define' on 'CustomElementRegistry': the name "acquisitions-menu" has already been used with this registry
4) Apply the second patch
5) Run `yarn js:build` in ktd (assuming ktd).
6) Reload the page without cached assets (Ctrl/Cmd + Shift + R) and verify that the uncaught error is gone.
7) Sign off.

There's an additional problem here I (and probably you) noticed. But that's unrelated to this bug.

Signed-off-by: David Nind <david@davidnind.com>

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>

koha-tmpl/intranet-tmpl/prog/js/vue/modules/islands.ts

@@ -107,6 +107,10 @@ export function hydrate(): void {
             }
 
             const component = await importFn();
+            if (customElements.get(name)) {
+                return;
+            }
+
             customElements.define(
                 name,
                 defineCustomElement(component as any, {