Handle existing identities in import script

If an identity already exists in greg, but is tied to a different
person, we tried to add it, causing an integrity error. We now check if
the value already exists. If it is tied to the person we are adding info
for, we simply update it. Otherwise we skip this value.

greg/management/commands/import_guests.py

@@ -68,7 +68,7 @@ class Command(BaseCommand):
             help="End date of roles. Default: today + 100 days",
         )
         parser.add_argument(
-            "--ignore_id_req",
+            "--ignore-id-req",
             default=False,
             action="store_true",
             help="Imports people without required ids if set",
@@ -205,10 +205,15 @@ class Command(BaseCommand):
             logger.info("role_created", role=role, person=person, sponsor=sponsor)
         return role
 
-    def upsert_identity(self, person: Person, id_data: dict) -> Identity:
-        """Add or update identity"""
+    def upsert_identity(self, person: Person, id_data: dict) -> Optional[Identity]:
+        """
+        Add or update identity
+
+        If the identity does not exist, we create it. If it exists on
+        the person we are working on, we update it. If it exists on
+        someone else we ignore it.
+        """
         match = Identity.objects.filter(
-            person=person,
             type=id_data["id_type"],
             value=id_data["external_id"],
         ).first()
@@ -224,14 +229,16 @@ class Command(BaseCommand):
             logger.info(
                 "identity_added", identity=identity.id, identity_type=identity.type
             )
-        else:
+            return identity
+        if match.person == person:
             identity = match
             if not identity.verified_at:
                 identity.source = id_data["source_system"]
                 identity.verified = "automatic"
                 identity.verified_at = make_aware(datetime.datetime.now())
                 identity.save()
-        return identity
+            return identity
+        return None
 
     def _has_required_id(self, id_data: dict) -> bool:
         """Check that we have at least one of the required ids"""