How to implement default entities in Drupal?

Question

PHP

There are many duplicate records (entities) of exchange rates for each region. Regions are taxa, just like monetary units, the connection with them is carried out using the fields field_currency_in, field_currency_out, field_region. There is no special rate for most regions, i.e. the exchange rate in these regions corresponds to the default regional exchange rate for that country.
As I said, the records are duplicated. It is necessary, probably, to redefine access to courses, i.e. if a record for the given region is not found, issue a default record or change duplicate records if the default rate has changed.
Show me the easiest way . I suspect changing many records instead of one is fraught with problems, and an additional field is needed that indicates that this is the default rate.