Gift Aid Declaration

I have inherited a database which is in serious need of cleaning. I have just received a donation with a new Gift Aid declaration. I want to add the new declaration to the constituents file. I have added all of the information such as the date of the declaration, that we have a written copy and that the donor pays tax etc. However, when I try to save the new declaration a pop-up appears saying "This declaration conflicts with an exisiting declaration. It could make previously ineligible gifts eligible for tax claim." Please help! How can I save this new declaration? The donors previosu declarations were made on 11 April 2008, 17 July 2009 and 22 June 2012. This one was made on 14 December 2017. I know he is a tax payer. The declarations made in 2008 and 2009 both have this recorded, but the declaration made in 2012 says 'unknown'. Does anyone know why this new declaration won't save? And, what I need to amend to be able to save it correctly? Many thanks.