Given the clarification in the comments, I am quite confident that this is simply a case of repetition for emphasis. The sentence in question is
Questo è il fatto... bello bello, chiaro chiaro e tondo tondo. Mio padre mi insegnò che è obbligo dare soccorso a mare
(Incidentally this sentence sounds a bit weird. It's not wrong but I would not consider it an example of "good Italian". Apparently it comes from some subtitle but I was unable to track down the movie in question. I have to assume that there is some visual context that explains it)
My interpretation is that it is a modification of the following
Questo è il fatto... Bello, chiaro e tondo
That we can roughly translate as
This is the long and short of it, without any possible ambiguity
(From chiaro e tondo, which means plainly, without mincing words, unambiguously, with an added bello for emphasis)
The speaker however repeats all the words in the expression twice in order to add further emphasis. This is not something I would commonly use. However the expression chiaro e tondo is quite common (ex: Te lo dico chiaro e tondo, I'm telling you plainly).
Just to add to your initial motivation for looking up this word: tono tondo is not a common Italian turn of phrase either. I can imagine using it to discuss some singer or musician (as in Her voice has a beautiful, round, tone in that passage) but that's about it, and even this is fairly uncommon.