With family ties set to be strained in this season's Champions League and Europa League, UEFA.com meets the brothers who took sibling rivalry to another level.
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The draw for this season's UEFA Champions League group stage may have caused a sharp intake of breath in the Hernández household, with brothers Lucas and Théo set to be on opposite sides as Paris take on Milan.
New Liverpool arrival Alexis Mac Allister also faces a reckoning with his past, with the Reds drawn against a Union Saint-Gilloise side featuring his older brother Kevin. UEFA.com meets the brothers who took fraternal rivalry to the highest levels of European football.
Florentin Pogba was adamant that he would not give his younger brother Paul an easy ride, when the centre-back's St-Étienne side drew Man United in the 2016/17 Europa League, explaining: "If he's on a run, and I have to tackle him, I'll tackle him. I'm not going to knock him out. But if I have to foul him, I'll do it."
Florentin reckoned the Pogba brothers were quite similar in terms of temperament and sense of humour, with one small exception: "Paul is a bit more tense, a bit grumpier on the pitch because he doesn't like losing." However, the public did not get to see that side of Pogba on this occasion; Zlatan Ibrahimović scored a hat-trick in the Old Trafford leg, while Henrikh Mkitaryan got the only goal in the return fixture in France.
Taulant and Granit Xhaka
Albania 0-1 Switzerland
01/11/16, UEFA EURO 2020 group stage
Arsenal 2-0 Basel
06/12/2016, UEFA Champions League group stage
Basel 1-4 Arsenal
06/12/2016, UEFA Champions League group stage
Kevin-Prince Boateng of Ghana met brother Jérôme's Germany at the 2010 and 2014 FIFA World Cups, while Steve Mandanda and goalkeeping sibling Parfait occupied opposite ends during an unofficial France friendly against DR Congo, but the Xhaka brothers' encounter was a European first.
"We've made a bet [on the result of the match] but have not yet decided on the reward," said Granit, who elected to represent his country of birth, while Taulant chose his parents' homeland. Hopefully, for Taulant's sake, all bets were off by the time they met again with their clubs in the 2016/17 Champions League.
Łukasz and Adrian Cieślewicz
B36 Tórshavn 1-2 The New Saints
01/07/2015, UEFA Champions League first qualifying round
The New Saints 4-1 B36 Tórshavn
07/07/2015, UEFA Champions League first qualifying round
"Mum has already said she won't come to the stadium, because she doesn't know which team to support," revealed Łukasz Cieślewicz after B36 drew his brother Adrian's TNS side, the Polish pair having been team-mates with the Faroese club in the Europa League the previous season.
"It's going to be fun to play against him," the former Brøndby player told UEFA.com ahead of the first leg. The matches did not prove much of a jolly for the then Faroese champions, though B36 captain Łukasz did at least score in the second leg in Wales – but only with his team already 4-0 down.
Thiago Alcántara and Rafinha
Barcelona 3-0 Bayern München
06/05/2015, UEFA Champions League semi-final
Footage surfaced of five-year-old Thiago and three-year-old Rafinha offering no quarter during a kickabout while their father Mazinho was playing at Valencia, but there was not much time for fireworks in their only UEFA encounter, Rafinha coming on after 87 minutes to face his brother – and, confusingly, another Rafinha – in Barcelona's 3-0 win. He did not make it off the bench in the return fixture.
The games caused their father a good deal of stress. "It's not even a toss-up for me," the 1994 World Cup winner said. "If I flipped a coin, it'd land on its edge. With a son on both sides, it's impossible for me to choose."
David and Philipp Degen
Young Boys 4-2 Stuttgart
01/12/2010, UEFA Europa League group stage
The only twin brothers to have met in a UEFA club competition, the Degens were thrown together after right-back Philipp was loaned from Liverpool to Stuttgart for the season, with forward David back in Switzerland with Young Boys after a spell in the Bundesliga with Mönchengladbach.
"We have played each other twice before," Philipp said, setting himself up for a fall ahead of their tussle. "I started on both occasions whereas he came on as a substitute. I won both games." His run came to an end on a snowy night in Berne, David netting the opening goal in a 4-2 win that sent his side to the round of 32.
Gabriel and Diego Milito
Barcelona 1-0 Inter Milan
28/04/2010, UEFA Champions League semi-final second leg
Barcelona centre-back Gabriel never got off the bench in the first leg of their 2009/10 Champions League semi-final, watching his team lose 3-1 to the soon-to-be European champions, and while he was able to face striker brother Diego in the return match, his victory was a hollow one.
The brothers had been in competition before back in their native Argentina, where Gabriel's Independiente used to encounter Diego's Racing Club, and the former recalled how passions boiled over in one infamous Buenos Aires derby. "The referee had to pull us apart because we kept on shouting terrible things at each other," he said. "It was fairly childish behaviour, like we were kids at home."
The Riises came into direct confrontation in the 2009/10 Europa League group stage, with left-sided John Arne running straight at right-sided midfielder Bjørn Helge. In the event, John Arne's Roma prevailed in the second of the two fixtures, though little brother Bjørn Helge's Fulham ended up reaching the final, losing to Atlético.
The elder sibling by three years, John Arne relished the prospect of taking on Bjørn Helge before that first meeting, saying: "I always look forward to playing against someone I know and I know my brother better than anyone. My brother and I are room-mates with the Norway team. Like any young player, he has lots of duties like carrying the balls and dealing with the kit. In our room we have various competitions which I can't go into detail about, but for us it's competition time again."
Erwin Koeman and the younger Ronald both figured in the Netherlands' 1988 UEFA European Championship-winning squad, but found themselves in opposition during the 1988 Super Cup as Erwin's Mechelen – winners of the 1987/88 Cup Winners' Cup – surprisingly overcame Ronald's PSV, then European champions.
On the pitch no brotherly love was lost. "If necessary, I will bring down Erwin," Ronald once said. "Maybe I go in even harder because he is my brother. I don't want him to beat me." Not that beating his sibling was ever much of a pleasure. "When I beat him in a board game or something he used to drive me mad because he acted like he didn't care," Ronald remembered. "I couldn't stand that."