The SAR Activity Review – Trends, Tips & Issues (October 2000) addresses repeated SAR filings on the same activity:
One of the purposes of filing SARs is to identify violations or potential violations of law to the appropriate law enforcement authorities for criminal investigation. This is accomplished by the filing of a SAR that identifies the activity of concern. Should this activity continue over a period of time, it is useful for such information to be made known to law enforcement (and the bank supervisors). As a general rule of thumb, organizations should report continuing suspicious activity with a report being filed at least every 90 days. This will serve the purposes of notifying law enforcement of the continuing nature of the activity, as well as provide a reminder to the organization that it must continue to review the suspicious activity to determine if other actions may be appropriate, such as terminating its relationship with the customer or employee that is the subject of the filing.
FinCEN’s SAR Activity Review information may be found here: https://www.fincen.gov/sar-activity-review-trends-tips-issues