Explanation for error message

Hi,

When I try to save a data, I am getting the following error message:

1 . [t_17143000000004266] null, message from server: "Duplicate entry '' for key 3".

How to find which is "Key 3" ?

NOTE: Moreover, in this entire form, I have only one field as Unique and the data I entered is definitely unique. So, this error makes me wonder whether there is any error in the application ?