Forms Authentication, doubling ReturnUrl parameters

I'm using Forms Auth and this is the current situation on when my returnUrl is incorrect

Say a user pops this URL into their browser since they had bookmarked this from a prior visit.


http://www.mywebsite.com/InvoiceDetail.aspx?OrgID=1&InvoiceID=1234


So forms auth directs them to my Login page but as soon as it does, their Url gets changed to: 

http://www.mywebsite.com/LogIn.aspx?ReturnUrl=%2fInvoiceDetail.aspx%3fOrgID%3d1%26InvoiceID%3d1234&OrgID=1&InvoiceID=1234


So once they do login, the url is not valid due to the duplicates in the URL string with OrgID and Invoice ID in the string twice. Why does it do this? I'm not setting the url anywhere so this is being done automatically somehow?


Now if a user logs in, and browses to that particular invoiceID address, and then logs out, they are given an encoded URL that's correct
http://www.mywebsite.com/ ...