Okay, this is what I found. Sorry for the format. I only went back the last four years, two years at 20 conference games (7 home/home opponents) and two years at 18 conference games (5 home/home).
Illinois:
2019-2020: Iowa, Maryland, Michigan, Michigan State, Northwestern, Purdue, Rutgers
2018-2019: Indiana, Minnesota, Nebraska, Northwestern, Ohio State, Penn State, Wisconsin
2017-2018: Nebraska, Wisconsin, Michigan State, Indiana, Rutgers
2016-2017: Indiana, Minnesota, Ohio State, Rutgers, Wisconsin
Indiana:
2019-2020: Maryland, Minnesota, Nebraska, Ohio State Penn State, Purdue, Wisconsin
2018-2019: Illinois, Iowa, Michigan, Michigan State, Northwestern, Purdue, Rutgers
2017-2018: Iowa, Minnesota, Michigan State, Illinois, Ohio State
2016-2017: Illinois, Iowa, Minnesota, Nebraska, Wisconsin
Iowa:
2019-2020: Illinois, Maryland, Michigan, Minnesota, Nebraska, Penn State, Purdue
2018-2019: Indiana, Michigan State, Nebraska, Northwestern, Ohio State, Rutgers, Wisconsin
2017-2018: Penn State, Indiana, Michigan, Ohio State, Minnesota
2016-2017: Indiana, Michigan, Michigan State, Penn State, Purdue
Maryland:
2019-2020: Illinois, Indiana, Iowa, Michigan State, Northwestern, Ohio State, Rutgers
2018-2019: Michigan, Minnesota, Nebraska, Ohio State, Penn State, Purdue, Wisconsin
2017-2018: Purdue, Penn State, Michigan State, Michigan, Northwestern
2016-2017: Michigan, Northwestern, Ohio State, Purdue, Wisconsin
Michigan:
2019-2020: Illinois, Iowa, Michigan State, Nebraska, Ohio State, Purdue, Rutgers
2018-2019: Indiana, Maryland, Michigan State, Minnesota, Northwestern, Penn State, Wisconsin
2017-2018: Ohio State, Iowa, Purdue, Maryland, Northwestern
2016-2017: Iowa, Maryland, Minnesota, Penn State, Purdue
Michigan State:
2019-2020: Illinois, Maryland, Michigan, Minnesota, Northwestern, Penn State, Wisconsin
2018-2019: Indiana, Iowa, Michigan, Nebraska, Ohio State, Purdue, Rutgers
2017-2018: Rutgers, Maryland, Indiana, Illinois, Wisconsin
2016-2017: Iowa, Ohio State, Penn State, Rutgers, Wisconsin
Minnesota:
2019-2020: Indiana, Iowa, Michigan State, Northwestern, Ohio State, Penn State, Wisconsin
2018-2019: Illinois, Maryland, Michigan, Nebraska, Purdue, Rutgers, Wisconsin
2017-2018: Nebraska, Indiana, Northwestern, Purdue, Iowa
2016-2017: Illinois, Indiana, Michigan, Northwestern, Rutgers
Nebraska:
2019-2020: Indiana, Iowa, Michigan, Northwestern, Ohio State, Rutgers, Wisconsin
2018-2019: Illinois, Iowa, Maryland, Michigan State, Minnesota, Penn State, Purdue
2017-2018: Minnesota, Wisconsin, Penn State, Illinois, Rutgers
2016-2017: Indiana, Northwestern, Penn State, Purdue, Rutgers
Northwestern:
2019-2020: Illinois, Maryland, Michigan State, Minnesota, Nebraska, Penn State, Purdue
2018-2019: Illinois, Indiana, Iowa, Michigan, Ohio State, Rutgers, Wisconsin
2017-2018: Penn State, Minnesota, Michigan, Wisconsin, Maryland
2016-2017: Maryland, Minnesota, Nebraska, Ohio State, Penn State
Ohio State:
2019-2020: Indiana, Maryland, Michigan, Minnesota, Nebraska, Penn State, Wisconsin
2018-2019: Illinois, Iowa, Maryland, Michigan State, Northwestern, Purdue, Rutgers
2017-2018: Michigan, Iowa, Rutgers, Penn State, Indiana
2016-2017: Illinois, Maryland, Michigan State, Northwestern, Rutgers
Penn State:
2019-2020: Indiana, Iowa, Michigan State, Minnesota, Northwestern, Ohio State, Rutgers
2018-2019: Illinois, Maryland, Michigan, Nebraska, Purdue, Rutgers, Wisconsin
2017-2018: Iowa, Maryland, Northwestern, Nebraska, Ohio State
2016-2017: Iowa, Michigan, Michigan State, Nebraska, Northwestern
Purdue:
2019-2020: Illinois, Indiana, Iowa, Michigan, Northwestern, Rutgers, Wisconsin
2018-2019: Indiana, Maryland, Michigan State, Minnesota, Nebraska, Ohio State, Penn State
2017-2018: Maryland, Rutgers, Michigan, Minnesota, Wisconsin
2016-2017: Iowa, Maryland, Michigan, Nebraska, Wisconsin
Rutgers:
2019-2020: Illinois, Maryland, Michigan, Nebraska, Penn State, Purdue, Wisconsin
2018-2019: Indiana, Iowa, Michigan State, Minnesota, Northwestern, Ohio State, Penn State
2017-2018: Michigan State, Purdue, Ohio State, Nebraska, Illinois
2016-2017: Illinois, Michigan State, Minnesota, Nebraska, Ohio State
Wisconsin:
2019-2020: Indiana, Michigan State, Minnesota, Nebraska, Ohio State, Purdue, Rutgers
2018-2019: Illinois, Iowa, Maryland, Michigan, Minnesota, Northwestern, Penn State
2017-2018: Nebraska, Purdue, Illinois, Michigan State, Northwestern
2016-2017: Illinois, Indiana, Maryland, Michigan State, Purdue
In my time researching, I found there are no locked rivals that play home-home every year which I find incredibly stupid. Duke-North Carolina play twice every year, it makes for a can't miss event on television, their fans love it, so why wouldn't the Big Ten try to do that for their top rivals as well? I understand competitive balance as it wouldn't exactly be fair to Michigan to have to play Michigan State and Ohio State twice a year, every year, while Maryland could get Penn State and Rutgers (this year excepted).
This may be changing with the move to a 20 game conference schedule. A 20 game conference schedule allows 7 home-home teams during the season so there is going to be one repeat between two years. We need next year's schedule to see if the same team will repeat for a third year but I haven't seen anything suggesting that.
Links:
https://bigten.org/news/2019/4/17/2019-2...kdown.aspx
https://btn.com/2018/04/19/big-ten-relea...breakdown/
https://btn.com/2017/08/16/view-the-full...-schedule/
https://btn.com/2015/02/19/big-ten-mens-...ce-season/