WASHINGTON -- The Internal Revenue Service warns that many taxpayers are mistakenly trying to double-dip on last year's federal tax rebate when they file their tax returns this year -- and the IRS is catching the errors.

A sampling of returns shows that 15 percent of early filers made mistakes in the section dealing with the rebate, the IRS said Friday. The errors were found using IRS software that checks for mathematical errors in returns.

The IRS sent taxpayers rebate checks as part of last year's economic stimulus package. Individuals received up to $600 and married couples $1,200, plus $300 for eligible children younger than 17.

The majority of taxpayers are ineligible for any more credits under the package. However, this year's tax returns include a section for the few taxpayers who are eligible for an additional credit.

Many taxpayers have incorrectly entered the amount of their rebate check on the line that calls for the amount of the credit they are claiming, the IRS said.

Instead, taxpayers should complete an accompanying worksheet to determine whether they qualify for an additional credit.

Taxpayers must know the amount of last year's rebate to complete the worksheet.

If taxpayers forgot the amount they received, they can get the information through a link on the IRS website at www.irs.gov or by calling the IRS at 866-234-2942.

Eligible taxpayers include those whose financial situations changed dramatically from 2007 to 208, taxpayers who were claimed as dependents for 2007 but not for 2008, parents who had an additional qualifying child in 2008 and people who did not file a 2007 tax return.