* Leon win 2-0 after extra time for 4-3 aggregate victory
* Leon become first club to win double in same season (Adds details, quotes)
By Carlos Calvo
MEXICO CITY, May 18 (Reuters) - Leon retained their Mexican league crown with a 2-0 second leg win on Sunday and a 4-3 aggregate victory over Pachuca in the final played between two teams owned by the same financial group.
Multi-millionaire magnate Carlos Slim holds a majority 30 percent share in the Pachuca Group which owns both finalists. Jesus Martinez, president of the group and the Pachuca team, lost to his son Jesus Martinez Murguia, who is Leon chairman.
Argentine striker Mauro Boselli was credited with the crucial goal that took the Clausura final, second leg into extra time in the 66th minute at the Hidalgo after Pachuca had won Thursday's first leg 3-2.
Pachuca's veteran former Mexico goalkeeper Oscar Perez dived to parry Boselli's header but the ball bounced in off his body.
Defender Juan Ignacio Gonzalez scored the clincher for Leon, led by Mexico World Cup captain Rafael Marquez and coached by Uruguayan Gustavo Matosas, well into extra time, heading in Luis Montes's corner.
"This is crazy, I'm very happy, my body's full of knocks but you just have to keep pushing and we came through. You can never stop trying and this became evident once again," Gonzalez told TV Azteca.
The champions lifted their seventh league title and became the first team to win the double of Apertura and Clausura in the same season.
UNAM Pumas won back-to-back titles with the 2004 Clausura followed by the following season's Apertura when they were coached by former Real Madrid and Mexico striker Hugo Sanchez.
Pachuca's Ecuador World Cup striker Enner Valencia was top scorer of the championship with 18 goals after his brace in the first leg of the final. (Writing by Rex Gowar in Buenos Aires; editing by Sudipto Ganguly)