Insert Into Creates Two Identical Rows

I have an "insert into" statement that creates two identical rows in a table, with this statement:

delete from [table] where [column] = @parameter
INSERT INTO [table]
([fields])
VALUES ([parameter values])

This is the code-behind that performs the insert:

Dim dbConn As New SqlConnection(strConn)
Dim cmd As New SqlCommand("sp_CreateUser", dbConn)
cmd.CommandType = Data.CommandType.StoredProcedure
cmd.Parameters.AddWithValue("@UserID", strUserID)
cmd.Parameters.AddWithValue("@UserName", strUserName)
cmd.Parameters.AddWithValue("@Email", strEmail)
cmd.Parameters.AddWithValue("@FirstName", strFirstName)
cmd.Parameters.AddWithValue("@LastName", strLastName)
cmd.Parameters.AddWithValue("@Teacher", strTeacher)
cmd.Parameters.AddWithValue("@GradYr", lngGradYr)
Using dbConn
dbConn.Open()
cmd.ExecuteNonQuery()
dbConn.Close()
cmd.Dispose()
dbConn.Dispose()
End Using

I wonder if it inserts twice due to a postback issue. Is there a way to stop two rows from being created in the first place with the same "insert into" statement? I'd appreciate any advice.