"nearly doubling" is correct and usable in written English.
You can use it to describe when something increases by almost, but not exactly, 100%. For example, "Sales increased by nearly doubling in the past month."
Prize money rose to $31.1 million, nearly doubling since 1989.
"You're talking about nearly doubling the greenhouse-gas emissions.
The stock soared, nearly doubling by the end of October.
Shanghai's population has reached 24m, nearly doubling in two decades.
Between 2001 and 2007, Americans went on an incredible borrowing binge, nearly doubling their household debt.
Like many other stocks in the home building industry, Lennar surged, nearly doubling in price.
Guangdong's population has grown dramatically since 1980, nearly doubling in size since then.
In Game 5, Boston still had 15 turnovers, nearly doubling the Knicks' 8.
The gift also includes four Ellsworth Kellys, nearly doubling this artist's paintings in the Whitney's collection.
Merrill's directors rewarded Mr. O'Neal by nearly doubling his pay package, to $28.1 million.
After nearly doubling over the previous five years, Mr. Ciervo said, prices have started to slip.
