InsertCommand.ExecuteNonQuery() And Violation Of PrimaryKey

Hey,

I have a page that inserts into a customers table in the DataBase a new customer account using this function:

Public Function InsertCustomers(ByRef sessionid, ByVal email, ByVal pass, Optional ByVal fname = "", Optional ByVal lname = "", Optional ByVal company = "", Optional ByVal pobox = "", Optional ByVal add1 = "", Optional ByVal add2 = "", Optional ByVal city = "", Optional ByVal state = "", Optional ByVal postalcode = "", Optional ByVal country = 0, Optional ByVal tel = "")

Dim result As New DataSet

Dim tempid As Integer

Dim conn As New SqlConnection(ConfigurationSettings.AppSettings("Conn"))

Dim Adcust As New SqlDataAdapter

Adcust.InsertCommand = New SqlCommand

Adcust.SelectCommand = New SqlCommand

Adcust.InsertCommand.Connection = conn

Adcust.SelectCommand.Connection = conn

sessionExists(email, sessionid, 1)

conn.Open()

If fname = "" Then

Adcust.InsertCommand.CommandText = "Insert Into neelwafu.customers(email,password,sessionid) Values('" & email & "','" & pass & "','" & sessionid & "')"

Else

Dim strsql As String

strsql = "Insert Into neelwafu.customers"

strsql = strsql & "(sessionid,email,password,fname,lname,company,pobox,address,address2,city,state,postalcode,countrycode,tel) values("

strsql = strsql & "'" & sessionid & "','" & email & "','" & pass & "','" & fname & "','" & lname & "','" & company & "','" & pobox & "','" & add1 & "','" & add2 & "','" & city & "','" & state & "','" & postalcode & "', " & country & ",'" & tel & "')"

Adcust.InsertCommand.CommandText = strsql

End If

Adcust.InsertCommand.ExecuteNonQuery()

Adcust.SelectCommand.CommandText = "Select Max(id) from neelwafu.Customers"

tempid = CInt(Adcust.SelectCommand.ExecuteScalar())

conn.Close()

Return tempid

End Function

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Now, I am getting an error:
Violation of PRIMARY KEY constraint 'PK_customers_1'. Cannot insert duplicate key in object 'customers'. The statement has been terminated.

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The customers table has as a primary key the 'email'.....

so plz can I know why am I getting this error ????

Thank you in advance

Hiba