我已经吃饭了晚饭:it's not about "less natural". It's totally WRONG! In chinese, there is a structure called 动宾结构. For example, 吃 is a verb, 饭 is a noun, we put 饭 after 吃 to describe / limit this verb, in other words, when we say 吃饭, we know that you are eating food, not fruit or any other things. You see, 饭 limit the verb 吃, it tells us what you eat. In this case, 吃 is a verb --- 动词, 饭 is a noun as an object --- 宾语, so it's called 动宾结构. 动宾结构 can be regarded as a single verb. For example, 我吃饭了. We say 吃饭 is the verb of this sentence. We don't say 吃 is a verb and 饭 is the object. So if you say 我已经吃饭了晚饭. It means that I have eaten food diner.... This is WRONG. When we talk, we may say 你吃了吗? grammatically this is wrong because 吃 is a vt, not vi. So it must be followed an object. But, as the most frequent used 动宾结构 with 吃 is 吃饭, we usually omit 饭. So when someone says: 你吃了吗? to you, you know that he is asking you if you have eaten food or not, instead of eating fruit or other things.