Add finalizedResult prop and update then method

lib/collector.js

@@ -16,6 +16,7 @@ class Collector {
   #controller = null;
   #signal = null;
   #timeout = null;
+  #finalizedResult = null;
 
   constructor(keys, options = {}) {
     const { exact = true, reassign = false } = options;
@@ -107,17 +108,19 @@ class Collector {
   }
 
   then(onFulfilled, onRejected = null) {
-    return new Promise((resolve, reject) => {
-      this.#fulfill = resolve;
-      this.#reject = reject;
-      if (!this.done) return;
-      if (this.#cause) return void reject(this.#cause);
-      if (!this.validate) return void resolve(this.data);
-      Promise.resolve(this.validate(this.data)).then((result) => {
-        if (result) resolve(result);
-        else resolve(this.data);
-      }, reject);
-    }).then(onFulfilled, onRejected);
+    if (!this.#finalizedResult) {
+      this.#finalizedResult = new Promise((resolve, reject) => {
+        this.#fulfill = resolve;
+        this.#reject = reject;
+        if (!this.done) return;
+        if (this.#cause) return void reject(this.#cause);
+        if (!this.validate) return void resolve(this.data);
+        Promise.resolve(this.validate(this.data)).then((res) => {
+          resolve(res ?? this.data);
+        }, reject);
+      }).then(onFulfilled, onRejected);
+    }
+    return this.#finalizedResult;
   }
 }