The United States enacted an estate tax in 1916, and when John D. Rockefeller, America’s first billionaire, died in 1937, his estate paid 70 percent. Since then, the rates have fluctuated, but this is the first time the tax has been repealed altogether.

The bonanza in tax savings for Mr. Duncan’s descendants is sure to be unsettling to those who have paid estate taxes on more modest wealth — until Jan. 1 of this year, it applied to any estate valued at more than $3.5 million, taxing only the money exceeding that threshold, or $7 million for a couple’s estate.

Although the tax affects only about 5,500 estates a year, it is such an incendiary issue that when Congress unexpectedly let it lapse at the end of 2009, financial advisers warned that it might play a macabre factor in the end-of-life decisions being weighed by heirs of elderly Americans. Some estate lawyers worried that tax considerations might prompt their clients to keep an ill relative on life support through the end of 2009 to get the favorable treatment — or worse, resist life-prolonging measures to hasten a relative’s demise before the end of 2010.

The one-year lapse in the estate tax was signed into law by President George W. Bush in 2001, an accounting quirk in his package of tax cuts. Although Democrats pledged to close that gap and reinstate a tax for 2010 when they took control of Congress, they failed to reach an agreement last December. The Senate Finance Committee is now trying to forge a compromise that would reinstate the tax, but even if that effort succeeds, it is unclear whether any changes might be retroactive and applied to those who have died so far in 2010.

Many lawyers say Mr. Duncan’s heirs have the means and motivation to wage a fierce court battle to challenge the constitutionality of any retroactive tax.