I'm reading おぼれかけた兄妹 by Areshima Takeo and I've come across the following phrase けれども二人がこっちに来るののおそいことおそいこと。Why this double のの? And what about こと? A man is saving a girl's life in the sea and the two are approaching the shore very slowly because of the rough sea.
The こと is a particle, expressing exclamation in particular.
As for のの, your understanding is fine. 来るのの is equivalent to 来ることが.
Putting together, the part translates to (their) coming is so slow!.