diff --git a/web/service/client.go b/web/service/client.go
index 47cda809..c8113f00 100644
--- a/web/service/client.go
+++ b/web/service/client.go
@@ -475,6 +475,18 @@ func (s *ClientService) Create(inboundSvc *InboundService, payload *ClientCreate
 		}
 	}
 
+	if client.SubID != "" {
+		var subTaken int64
+		if err := database.GetDB().Model(&model.ClientRecord{}).
+			Where("sub_id = ? AND email <> ?", client.SubID, client.Email).
+			Count(&subTaken).Error; err != nil {
+			return false, err
+		}
+		if subTaken > 0 {
+			return false, common.NewError("subId already in use:", client.SubID)
+		}
+	}
+
 	needRestart := false
 	for _, ibId := range payload.InboundIds {
 		inbound, getErr := inboundSvc.GetInbound(ibId)
@@ -646,6 +658,18 @@ func (s *ClientService) Update(inboundSvc *InboundService, id int, updated model
 		}
 	}
 
+	if updated.SubID != "" {
+		var subCollision int64
+		if err := database.GetDB().Model(&model.ClientRecord{}).
+			Where("sub_id = ? AND id <> ?", updated.SubID, id).
+			Count(&subCollision).Error; err != nil {
+			return false, err
+		}
+		if subCollision > 0 {
+			return false, common.NewError("Duplicate subId:", updated.SubID)
+		}
+	}
+
 	needRestart := false
 	for _, ibId := range inboundIds {
 		inbound, getErr := inboundSvc.GetInbound(ibId)