An error from my past projects

When i build todo list in the web, i need to distinguish between users in the database. So I should post the information of users, and get the information of the user. The problem is that when I add an account to the database, two data will be add to database. I guess this question is that when I post the information to the database, the project post twice. So I look for the code of post, and I find two code of post. At last I delete one to solve the question. I analysis the reasons according to the failure.