Return an error if an invite for a confirmed remote cluster is received (#28943)

Co-authored-by: Caleb Roseland <caleb@calebroseland.com>

server/platform/services/remotecluster/error.go

@@ -3,7 +3,14 @@
 
 package remotecluster
 
-import "fmt"
+import (
+	"errors"
+	"fmt"
+)
+
+var (
+	RemoteClusterAlreadyConfirmedError = errors.New("the remote cluster has already been confirmed")
+)
 
 type BufferFullError struct {
 	capacity int

server/platform/services/remotecluster/recv.go

@@ -62,6 +62,10 @@ func (rcs *Service) ReceiveInviteConfirmation(confirm model.RemoteClusterInvite)
 		return nil, fmt.Errorf("cannot accept invite confirmation for remote %s: %w", confirm.RemoteId, err)
 	}
 
+	if rc.IsConfirmed() {
+		return nil, fmt.Errorf("cannot accept invite confirmation for remote %s: %w", confirm.RemoteId, RemoteClusterAlreadyConfirmedError)
+	}
+
 	rc.SiteURL = confirm.SiteURL
 	rc.RemoteToken = confirm.Token