It would appear that you imported data into your file with a contact ID, but did not update the "next serial value" setting when you did so. Thus, each new record you create is getting a contact ID that duplicates the contact ID of an existing record and thus matches to the notes records for that other contact.
I'm guessing that you are referring to the Contact mangement starter solution here (there are many, many contact managers out there...)
If so, put the k_ID_Contact field on your record detail layout. Select Show All Records unless it is greyed out. Then select Sort Records and sort your records by K_ID_Contact in descending order.
Make sure you are on the first record and note the value shown in K_ID_Contact.
Now open Manage | Database | Fields, find K_ID_Contact and double click it.
Select the auto-enter tab in the field options dialog that has popped up and find the box named "next value".
Change the value in this field to be at least 1 larger than the maximum value you saw in the K_ID_Contact field.