In versions of CMS prior to 11.12.26 there is a feature for "Exclude Goods received amounts" when doing a year-end type letter.
Using this feature will cause duplicate amounts if the payment as split into multiple categories.
Because of this bug, this option has been removed in version 11.12.26. If you are using a version prior to 11.12.26 you should not use this option (Exclude Goods received amounts) if you have any donations that were split into multiple categories.
We apologies for the inconvenience and are working on a more rounded solution to this issue in future releases.
For more information about Year End letters, please see the following KB articles.