Your data is like I suspected. Since you are updating this table based on the ClinicCode you will update all of the records in this table where the clinic code matches. The ClinicCode is not unique to an individual record so any updates based on this will update all the where there is a matching clinic code.
To get past this you will need to have a unique identifier for the table you are trying to update. The best way to do this is to add an id column and make an an auto incrementing primary key for the table. Then you would use this id reference as the unique identifier when you are updating a record. Only one record should match for this id.