There is an option in the Company Profile to automatically incremement invoice numbers. So unless the invoice number is edited on the invoice (or the company profile value is decremented), this will ensure unique numbers.

Technically, it is not an error to post the same document number to different customers (which is why we don't prevent or warn in this scenario), but can understand why you wouldn't want that. Thus the increment option.

Posting does check for duplicate document numbers for the same customer and prevents it. I believe the batch listing report also looks for this.