A US judge has thrown out one of three charges against Jose Padilla, the US citizen who was held for more than three years as an "enemy combatant".
Judge Marcia Cooke said the charge of "conspiring to murder, kidnap and maim persons in a foreign country" referred to the same actions as other charges.
As such, it violated a US ban on trying people for the same offence twice.
Suspected al-Qaeda conspirator Mr Padilla was arrested in May 2002 and held in military custody until January.
The US government originally indicated it suspected him of plotting to set off a radioactive "dirty bomb" in a US city, but he has never been charged over that alleged plot.
After protracted legal battles his case was moved to a civilian court in November 2005.
His case was then added to that of two other men already charged with three terror-related offences.
But on Monday, Judge Cooke said one was redundant, and that "the government has charged a single conspiracy multiple times, in separate counts, when in law and in fact, only one [alleged] crime has been committed", the Miami Herald quoted her as saying.
The three men - Mr Padilla, Adham Amin Hassoun, and Kifah Wael Jayyousi - still face charges of providing material support to terrorists and conspiring to do so.
Their trial is expected to begin next year.
Mr Padilla, a former Chicago gang member and a convert to Islam, was arrested following a trip to Pakistan.
His military detention was heavily criticised by civil rights groups and the case was seen as a test of the limits of the US government's anti-terrorism powers.
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