fix remove duplicated list elements when size are same, for issue #2944

core/src/main/java/com/alibaba/fastjson2/reader/FieldReaderCollectionMethodReadOnly.java

@@ -75,6 +75,11 @@ public void accept(T object, Object value) {
         }
 
         Collection values = (Collection) value;
+
+        if (values.size() == collection.size()) {
+            return;
+        }
+
         for (Object item : values) {
             if (item == null) {
                 collection.add(null);

core/src/test/java/com/alibaba/fastjson2/issues_2900/Issue2944.java

@@ -0,0 +1,78 @@
+package com.alibaba.fastjson2.issues_2900;
+
+import com.alibaba.fastjson2.JSON;
+import com.alibaba.fastjson2.JSONObject;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue2944 {
+    @Test
+    public void test() {
+        List<Employee> employees = new ArrayList<>();
+        employees.add(new Employee(0, "John"));
+        employees.add(new Employee(1, "Jane"));
+        employees.add(new Employee(2, "Bob"));
+        Department department = new Department("dev", employees);
+
+        String payload = JSON.toJSONString(department);
+        JSONObject jsonObject = JSON.parseObject(payload);
+        Department parsed = jsonObject.toJavaObject(Department.class);
+        assertEquals(payload, JSON.toJSONString(parsed));
+    }
+
+    class Department {
+        private final String name;
+        private final List<Employee> employees;
+
+        public String getName() {
+            return name;
+        }
+
+        public List<Employee> getEmployees() {
+            return employees;
+        }
+
+        public Department(String name, List<Employee> employees) {
+            this.name = name;
+            this.employees = employees;
+        }
+
+        @Override
+        public String toString() {
+            return "Department{" +
+                    "name='" + name + '\'' +
+                    ", employees=" + employees +
+                    '}';
+        }
+    }
+
+    class Employee {
+        private final Integer id;
+        private final String name;
+
+        public Integer getId() {
+            return id;
+        }
+
+        public String getName() {
+            return name;
+        }
+
+        public Employee(int id, String name) {
+            this.id = id;
+            this.name = name;
+        }
+
+        @Override
+        public String toString() {
+            return "Employee{" +
+                    "id=" + id +
+                    ", name='" + name + '\'' +
+                    '}';
+        }
+    }
+}