SUSHI should warn on path overwrites in the same instance

Given:
```
Instance: MyInstance
InstanceOf: MedicationRequest
status = $ConditionClinicalStatusCodes#active "Active"
status = $MedicationRequestStatusCodes#active "Active"
```
SUSHI should emit a warning. While the above is technically allowed, it's 99.999% sure to be a mistake. The only case where something like this might be intentional is when a property is set by an inserted ruleset but then overridden later. SUSHI needs to be able to distinguish that valid use case and _not_ emit a warning for that. But for the use case shown above, it's definitely warnable.

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/MedicationRequest.20No.20Error.20for.20Multiple.20Status