I can guess that You are inserting same value in your primary key field. SO I will be suggest to you that you should set is identity field --Yes and set seed value 1 and auto increment 1.
CREATE TABLE Persons
(
ID int IDENTITY(1,1) PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
Thanks.