Added duplicated trigger validation (#3124)

i18n/en.json

@@ -299,6 +299,10 @@
     "id": "api.command.admin_only.app_error",
     "translation": "Integrations have been limited to admins only."
   },
+  {
+    "id": "api.command.duplicate_trigger.app_error",
+    "translation": "This trigger word is already in use. Please choose another word."
+  },
   {
     "id": "api.command.delete.app_error",
     "translation": "Inappropriate permissions to delete command"
@@ -1855,6 +1859,10 @@
     "id": "api.webhook.create_outgoing.disabled.app_error",
     "translation": "Outgoing webhooks have been disabled by the system admin."
   },
+  {
+    "id": "api.webhook.create_outgoing.intersect.app_error",
+    "translation": "Outgoing webhooks from the same channel cannot have the same trigger words/callback URLs."
+  },
   {
     "id": "api.webhook.create_outgoing.not_open.app_error",
     "translation": "Outgoing webhooks can only be created for public channels."