fix: check that component wasn't instantiated (#10466)

fixes #10454

.changeset/gentle-owls-hug.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: check that component wasn't instantiated in `connectedCallback`

packages/svelte/src/runtime/internal/Component.js

@@ -227,7 +227,7 @@ if (typeof HTMLElement === 'function') {
 			if (!this.$$c) {
 				// We wait one tick to let possible child slot elements be created/mounted
 				await Promise.resolve();
-				if (!this.$$cn) {
+				if (!this.$$cn || this.$$c) {
 					return;
 				}
 				function create_slot(name) {