Some Americans are getting a second Social Security check today. Here's why.

About 1 in 10 Social Security recipients will receive a double December payment, with their second monthly check arriving on December 29. 

That's because of a quirk in the system for the Social Security Administration's Supplemental Security Income (SSI) program, which provides support for disabled people and older Americans with low incomes. 

Typically, SSI recipients receive their payments on the first of each month, which means the program's 7.5 million enrollees received their December payment on December 1. But because January 1 is a holiday, SSI recipients are getting their January check on the last weekday before the New Year, with deposits landing on December 29.