In this example I only used an %Extra_key, but since we're working with a facts table and dimensions setup this is actually multiple keys.
One of these keys is a %Date_key.
The row that should be flagged would the one that coincides with the earliest instance of the Incident. Datasource_B (The table with the Incident dimension) includes a SysCreated field which during the load statement would be turned into a %Date_Key for loading into the facts table only (to avoid a synthetic key)
Sadly though... Theoretically it is possible for 2 rows in the Facts table to have the same %Incident_key and %Date_key. So I'd rather create something that would simply load it in once per Incident and date combination. All that matters is that the earliest date per incident gets flagged, so that no double flags exist.
So essentially I'm loading the _Flag_IncidentNew into the Facts table using the Incidents table in which the %Incident_key is distinct, where during this load I'm transforming one of the columns in the Incidents table into a %Date_key.
In the actual situation (non-example) every %Incident_key gets flagged once.