"to give him a double portion": A double portion of what each brother receives? Or a double portion of the entire property? You reason thus: Just as he inherits with one brother, and he inherits with five, then just as when he inherits with one he inherits a double portion of one, so, when he inherits with five, he inherits a double portion of one.
Or, go in this direction: Since he inherits with one and he inherits with five, then just as when he inherits with one, he inherits a double portion of all the property, so, when he inherits with five, he inherits a double portion of all the property. It is, therefore, written "then it shall be on the day that he causes his sons to inherit." "inheritance to sons" (and not "inheritance of property") is repeated.