On the fafsa I accidentally put down that my mother both received and gave $5,975 for child support last year- when she had only received that much. My school of choice offered me a rather nice financial aid package (my mother’s income is $29,000 a year so I expected this, but not this much), and they want me to submit another form about child support. I realized the mistake just a day ago… Now I don’t know what to do, any advice? 
Call the FA office and talk to them. They probably already caught the error.
You submit the form accurately and tell your schools aid office that you accidentally entered the figure twice. Mistakes like his happen.
First resubmit the FAFSA with correct information. Then contact the FA office to tell them you made an error, and have just resubmitted your financial info.