26 CFR §1.1016-6

1. (a)Adjustments must always be made to eliminate double deductions or their equivalent. Thus, in the case of the stock of a subsidiary company, the basis thereof must be properly adjusted for the amount of the subsidiary company's losses for the years in which consolidated returns were made.
2. (b)In determining basis, and adjustments to basis, the principles of estoppel apply, as elsewhere under the Code, and prior internal revenue laws.