Is there any mechanism to prevent an author from cross-sending a manuscript?
The only mechanism is that being found out is typically both rather embarrassing and, in case of pre-tenure researchers, can also be very detrimental to career development of the researchers in question. 

Further, note that being found out is in many fields not as unlikely as you might think. For many more narrow topics, there are only so many researchers out there that are "natural candidates" for reviewing a given paper. Despite not being very senior yet, it has already happened to me that I received basically the same paper twice for review from different (in that case) conferences.