Cách hỏi "Đã ăn cơm chưa?"

1/ Đã ăn cơm chưa?
Have you eaten yet?

2/ Đã ăn cơm trưa chưa?

In BE it is considered incorrect to use "yet" with "did" or any other verb in the preterite. However, it is quite common in American speech.

In BE, the preterite is used for an action completed in the past. 

Have you had your lunch [yet]?
Yes, I've had my lunch.
Yes, I had lunch at 12.
No, I haven't had lunch [yet].
I would use "did" if talking about some time in the past.

Did you have lunch yesterday?
No, I was so busy, I didn't have time to stop for lunch.
Yes, I had lunch with the boss. She took us all to lunch at the new Thai restaurant.

3/ 

If you are talking about a common, or regular occurrence, you can say
"Do you have your lunch in the canteen every day?"
"Do you have your lunch at 12 o'clock on Monday?"

 https://forum.wordreference.com/threads/have-you-had-your-lunch.615438/