While working on a php insert-record form I was getting the error:

#1062 - Duplicate entry '127' for key 1 

Thanks to jc_armor for the answer to this:
"quick question, what's the Data Type of your ID field? It could be a reason that the data type can only handle up to 127 :) "

Changing the field type to "unsigned INT" as recommended in the forum message made the error disappear.

http://dev.mysql.com/doc/refman/5.1/en/numeric-types.html">MySQL Reference Manual: data types: numeric types