Spokesman Jason MacDonald says in an email that the PMO has responded "fully and freely" to requests for assistance from RCMP investigators.
He adds the Mounties have made it clear they don't believe Harper has any knowledge of Duffy's alleged wrongdoing and that there would be no reason for the prime minister to be involved should Duffy's defence team attempt to have him testify.
The RCMP announced last week it was charging Duffy with 31 criminal counts related to his expense claims, accusing him of misspending more than $200,000.
The charges stem from the disgraced senator's housing and travel expenses, and a $90,000 payment from Nigel Wright, the former chief of staff to Harper.
Duffy has denied any criminal wrongdoing.
A new court filing detailing the charges accuses Duffy of charging taxpayers for a personal trainer, a makeup artist and for personal travel to funerals.
The 68-year-old senator is scheduled to appear in court on Sept. 16.
In April, the RCMP concluded there wasn't enough evidence to charge Wright. The prime minister had previously insisted Duffy repay his expenses, but the senator said he didn't have the money.
Wright then stepped in and paid $90,000 to Duffy to help him cover his expenses.
The expense scandal has dogged the Harper government for more than a year. The next federal election is expected to be held in the fall of 2015, though Harper could call a spring vote.
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