In this article, it is said that:
If there’s a verb 了 at the end of a sentence that also needs a sentence 了, the two 了s just combined into one.
They also provided some examples.
However, these examples are not clear because they don’t show us how the sentences were before the two 了s merged.
我吃了。 他去了。 你说了。
Due to the unclear explanation + examples, I find it difficult to understand what they mean.
How was “我吃了” before the 了s merged? Was it 我吃了了? If yes, why do we need two 了s here? Is it to show that the subject has completed the action of eating and is now full (there’s a change of their condition)?