One thing I'm sure of is they really did get married a second time. I did a marriage search in Clark County Recorder's website. There were 2 records - almost a year apart. I am pretty sure they had no idea that it could be considered fraud - they even did it in the same county. Both names appear exactly the same.
We have established that, whether or not it's the lawyer's advice, they have committed fraud. It may not be a big deal since it doesn't harm anyone, except that they may present the 2nd marriage certificate to USCIS. If they have already done so, what are the chances that USCIS will find out about the first one? Name check, perhaps? If they haven't filed and later submits the first certificate, do they still have to tell USCIS about the 2nd one or do they have to do something to undo the 2nd marriage?
I really don't know whether the 2nd marriage is fraud or not. But, sending the certificate to the USCIS and applying for a greencard based on that marriage date is fraud. I'm not going to speculate as to the chances of getting caught. As for whether they should or should not tell the USCIS, I think they now need a competent lawyer to give them that kind of advice.
I have never heard of anything like this before. It must be exceptionally rare. But I do know that when a person is already married and then files for another marriage, the second marriage is generally automatically void in the eyes of the law. What I don't know is if there is a formal way to have the second marriage formally declared void in Nevada. I mean, there must a certain number of people who got drunk and got "married" in Vegas even though they were already married to someone else. I would imagine there is some kind of process for this kind of thing. But, if you get married to the same person twice, I don't know what a person can do. I mean, if you have the second marriage annulled, what effect does that have on the first marriage to the same person? I don't know. These questions are far beyond my competence.