If a NetSuite record is created from another NetSuite record, the internal ID of the newly created NetSuite record is initially the same as the internal ID of the NetSuite record it was created from, until the new record is saved.
Example: NetSuite Invoice and Invoice Lines to Salesforce Custom Invoice Header and Invoice Lines
If you click Bill Remaining on a NetSuite Sales Order, an Invoice is created. Before saving that newly created invoice, the internal ID of the Invoice header and lines are the same as the Sales Order that created it. After saving the Invoice, it'll have its own unique internal ID.
In the Settings for the Export out of NetSuite, the "Reload Record Before Export" checkbox needs to be checked in order to prevent the initial internal ID on the Line Level from syncing over to Salesforce. If it is not checked, in our example, the internal ID of the Sales Order Lines that created the Invoice will sync over instead of the Internal ID of the Invoice Lines. Then when the Invoice attempts to sync again from an update, it'll create additional Invoice Line records in Salesforce with the correct Internal IDs.