Retrials are not particularly great for the prosecution.
Remember in a criminal case, the prosecution has the burden of proving all the elements of each and every charged crime beyond a reasonable doubt. This is highest burden in our law at trial. One of the tools that a prosecutor has is keeping his trial strategy secret (as part of attorney work product) until the opening statement is delivered. During a prosecutor's opening statement, the defendant and his counsel hear, for the first time, how the government's proof will be introduced, through witnesses and other evidence, against a defendant. Yes, the defense attorney may already have strong hunches about what the evidence will be but an opening is the prosecutor's road-map of how the government intends to obtain a guilty verdict against a criminal defendant, like Roger Clemens.
Here, however, Clemens' attorneys have already heard the prosecutor's first opening statement in the original trial. This means that the government's hand has been officially "tipped."
It may be a small advantage but knowing the precise evidentiary road-map against your client is good fo the defense. For now, Clemens' legal team has a quick strike one on the prosecution.