I assume you mean in the USA specifically. No, it is not unprecedented for grad students to have a union and a union contract. It has been slowly gaining steam in the USA over the past 20 years -- as the US NLRB keeps changing it's mind about whether graduate students have the legal right to protections to organize unions (and sometimes different for public vs private universities).
I went looking for a list, and found the wikipedia article has some kind of old statistics (anyone wanna try to find newer ones and update it?), but let's you know it's not unprecedented: "As of 2014, there are at least 33 US graduate employee unions, 18 unrecognized unions in the United States, and 23 graduate employee unions in Canada," and "Since 2000, more than twenty campuses have unionized." —https://en.wikipedia.org/wiki/Graduate_student_employee_unio...
I haven't found a complete list, but some mentioned in the wikipedia article are Oregon State University, Michigan State University, Temple University, the University of Washington, and Columbia.
They are still more common at public than private schools, and I think it may be that none of the Ivy League universities have graduate student unions? MIT is not technically an Ivy, but is usually considered in the same tier of prestige/funding/quality. So in that sense MIT unionization is ground-breaking.
When I was a grad student at a public university, we were affiliated with a larger union (CWA, Communication Workers of America IIRC). So not unheard of I guess.
Exactly what benefits we got I don’t remember, but things got interesting once when CWA was talking about striking :)
