Using Automatic Entity Label how to add number if title is already used (unique)
You can add node id to the label. It will make labels unique anyway.
For example

[node:field_first_name] [node:field_last_name] #[node:nid]


Please note, that there is an issue in module that prevents [node:nid] token from working.
https://www.drupal.org/project/auto_entitylabel/issues/2920695

But there is simple patch available in this issue to fix this. So this should be resolved soon I hope. Until that you need to apply patch manually.
https://www.drupal.org/files/issues/2018-11-11/auto_entitylabel-support_entity_id-2920695-31.patch