A beraita (Tosefta) presents the idea of enhancement to the property that takes place after the death of the father - for which the first-born does not get the double-portion. But that's a point that needs refinement. It may depend who made those improvements. Plus, the question of who pays the father's debt - that is, as a double portion, or not? Also, more on the first-born not taking a double-portion when a loan was returned to the estate. But who supports that view?