What must be configured in order to keep two static routes to the same destination in the routing table?

While having the same priority is necessary for multiple static routes to the same destination to be active, it is not sufficient; the administrative distance must also be identical for them to both exist in the routing table.

FortiGate will keep multiple static routes to the same destination in its routing table only if they are configured with both the same administrative distance and the same priority, allowing for equal-cost multi-path routing or explicit failover.

Having the same administrative distance is necessary, but without also having the same priority, only the route with the highest priority (lowest numerical value) would be selected if distances were equal.

The term 'metric' is primarily used by dynamic routing protocols to determine the best path, whereas FortiGate's static routes use administrative distance and priority for selection and redundancy.