Probably you deleted one of the records before. So, if you are in a development period please go to phpmyadmin and clear the datas in the table. Then go to the Settings sections (top menu of phpmyadmin) and set
AUTO_INCREMENT as 0, in order to start from 1, then try posting the form again.
(If I'm not wrong Auto_INCREMENET will be 0 for the start. If not, then insert 1

)
The second option: you can add+1 to your id within your php codes. But I'd go with the first one.