When creating instances from a MARC file that already has 999 ff, the new 999 ff is wrong

Overview: Sometimes a user may want to export a file of records from one FOLIO tenant and then import into another to create records in that second tenant. Or they might want to export a file of records and then import them to create duplicate instances and SRS MARC in the same tenant (this is more of an edge case). MARC records exported from SRS have an existing 999 ff with the Instance and SRS UUIDs. When imported to create new Instances and SRS MARCs, the 999 field gets messed up.

Steps to Reproduce:

1. Log into FOLIO-snapshot-load as diku_admin
2. Go to Data Import
3. Load the attached file, using the secret button
4. Go to Inventory and look up one of the instances
5. View source for that instance
6. View the 999 ff field

Expected Results: There should be one $i and one $s in the 999 ff field. If the incoming file already has a 999 ff, and the action is to create instances, remove the incoming 999 ff and its data, and replace with a new 999 ff and new UUIDs.

Actual Results: There are two (the ones that came from the original MARC file, and the ones that were created when the file was imported.

Additional Information: See attached video and MARC file

A-M checked with librarians on preferred resolution; go with # 2

1. If the incoming file already has a 999 ff, disallow creation of instance (but would that make it harder to share records across tenants. On the other hand, if the create failed, the user could edit the file to remove the 999s and resubmit)
2. If the incoming file already has a 999 ff, and action is to create instances, remove the incoming 999 ff and its data, and replace with a new 999 ff and new UUIDs (but what if the user made a mistake and really wanted to update instead of create?)
3. Move the old 999 ff to 999 aa and do not fail