MM-16949 Fix posts returned from unread API when the two different list contains parent post and comments (#11617)

* fix posts returned from unread API when the list contains parent post/s

* add ExtendAll to PostList and update test per feedback

* revert unintentional change to the other test and fix comment

* update the existing postlist.Extend, filter unique values and update unit tests

model/post_list.go

@@ -95,13 +95,29 @@ func (o *PostList) AddPost(post *Post) {
 	o.Posts[post.Id] = post
 }
 
-func (o *PostList) Extend(other *PostList) {
-	for _, postId := range other.Order {
-		if _, ok := o.Posts[postId]; !ok {
-			o.AddPost(other.Posts[postId])
-			o.AddOrder(postId)
+func (o *PostList) UniqueOrder() {
+	keys := make(map[string]bool)
+	order := []string{}
+	for _, postId := range o.Order {
+		if _, value := keys[postId]; !value {
+			keys[postId] = true
+			order = append(order, postId)
 		}
 	}
+
+	o.Order = order
+}
+
+func (o *PostList) Extend(other *PostList) {
+	for postId := range other.Posts {
+		o.AddPost(other.Posts[postId])
+	}
+
+	for _, postId := range other.Order {
+		o.AddOrder(postId)
+	}
+
+	o.UniqueOrder()
 }
 
 func (o *PostList) SortByCreateAt() {