Most of the integrations make use of the External ID field as their 'link' from their External Site to NetSuite.
However, specifying certain External IDs can sometimes cause DUP_RCRD errors.
To resolve this, confirm that the External ID is not used for the other 'related' transaction types.
For example, if you are trying to set an External ID for a Sales Order, perform a search to check if there are other transactions (eg. Purchase Order, Item Fulfillment, Cash Sale, etc.) that make use of the said External ID.
For Customer records, perform a search to check if there are other entities (eg. Employee, Vendor, etc.)
Best practice indicates that we should use a prefix of some sort to be able to identify record types through their IDs. This will also resolve the DUP_RCRD error that users might encounter.