let x1, x2 be set ; :: thesis: {x1,x1,x2} = {x1,x2}
thus {x1,x1,x2} = {x1,x1} \/ {x2} by Th43
.= {x1} \/ {x2} by Th69
.= {x1,x2} by Th41 ; :: thesis: verum