You can use two sets of relationships for this.
Use one based on the importID field to temporarily establish a link. Then update the PID_fk field after import so that you can then rely on the internally generated Primary key in your People table to keep your records properly related.
Just after Import, the imported records form the current found set.
You can then use Replace field contents and this "import only" relationship based on the ImportID numbers to copy the value of PID from People to the PID_fk field in History.
Using additional table occurrences you can not only have separate and different relationships between the same two tables, but if you use the Anchor Bouy method, you can have a layout just for importing your history records which is based on a table occurrence in Manage | Database | Relationships that links to People by the ImportID fields. You can then use this layout for importing the data followed by a replace field contents action to add the missing PID number to PID_fk.
Your regularly used layouts need not refer to this same table occurrence and they can then utilize the PID based relationships.
lovely. thnaks phil as always. I needed the push to see a bit of redundancy might aid clarity and using "temporary" layouts for looking at data.
Nothing you can do I suppose about the fact that in the old file the UI's were not in fact unique in about a dozen cases. <s> fortuantely not many and the temp layouts help make the handwork easy.