Commentary for Kiddushin 122:5

The Talmud critiques the comparison. In the mishnah, the doubling is necessary to teach that if they don’t fulfill the condition, they will not receive any inheritance, neither in Canaan or in Gilad. Here, in the parable, the doubling is effective only in respect of the rest of the estate. He would have received that portion of the field in any case, whether he fulfilled the condition or not. This implies that without the doubling the tribes would have inherited in the Gilad even without the double formulation.