Suggest adding logic to check for duplicates when adding products

[Guangyaochenyu]

When I try to simulate repeated buy something, maybe found some inappropriate logic.

Here are some of my ideas to deal it.

Your code has given me lots of help. Thank you very much. And looking forward to your reply.

[JasonLeeForOnly]

这个特性非常需要，可以PR一下。 This feature is very needed, you can do a PR.