It's just a perference. In your question, 我已经吃了晚饭 is more natural than the other one. As I see, that's because we simply don't repeat "饭" twice. But there are conditions when both 吃饭 and 饭 are right. (你吃了吗？& 你吃饭了吗？are both right. ) 这只是一个习惯偏好。 你的问题里面，”我已经吃了晚饭“比起另一个更自然。 在我看来，不需要把“饭”重复两次。 但是有些情况两者都可以使用，比如：你吃了吗？& 你吃饭了吗？

"吃" это краткая форма от "吃饭"

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.