The Liberty Committee, a libertarian organization led by former Paul aide David James, said in an April 16 letter that about two-thirds of the 63 airline tickets for which the group reimbursed Paul also were paid for by taxpayers.
"In short, this practice of double or duplicate billing enriched you while draining funds intended for legitimate projects," the letter read.
Paul spokesman Jesse Benton said James is "pursuing a personal grudge" against Paul and that Paul would be "happy" to review the allegations, Roll Call reported Tuesday.
Paul announced Monday he was suspending actively campaigning for the GOP presidential nomination but would still campaign for delegates at state conventions.
Roll Call reported in February that Paul was paid twice on several occasions for flights between Washington and his congressional district. He was reimbursed from taxpayers and from political and non-profit organizations he controlled, public records and Paul's credit card statements indicated. In all, Roll Call discovered 26 flights from 1998 to 2005 for which documentation indicated double payments.
A Liberty Committee audit revealed more flights it said were reimbursed twice, raising the number of such instances to 52, Roll Call reported Tuesday.
James said the Liberty Committee was reviewing its legal options for repayment in court, including researching whether the statute of limitations expired.
Citizens for Responsibility and Ethics President Melanie Sloan said the House Ethics Committee should investigate the matter.
The April 16 Liberty Committee letter demanded repayment within 10 business days. In a letter dated April 26, Paul said his records didn't indicate "any mistaken reimbursements, but I do take these matters very seriously. Please send me copies of all the record [sic] you refer to -- invoices, receipts and tickets -- and I will have my staff review them thoroughly to determine if any mistakes were in fact made."
Benton said James "has not sent any records or documentation of any kind to back up his claims of decade-old double reimbursement, only a letter demanding $20,000."
Since the story broke, Rep. Mike Quigley, D-Ill., introduced a resolution that would strengthen travel reporting requirements for House members.