There is no immediate problem with a person having duplicate constituent records and/or multiple Bidder IDs. They can still bid on the auction; however, they may need to re-enter their credit card if they used a different email address to purchase their event tickets than the one they used to create a new portal account.
If bidder numbers are pre-assigned and you are doing something like a live auction or paddle raise for donations, at the end of the evening you may find that a person has auction items assigned under two bidder ID numbers. You would need to check the items related to each constituent/bidder ID separately. For example, if Margot Smith is bidder ID 103 and Maggie Smith is the same person, but she created a new bidder with ID 215, you would check out all the items for Bidder 103 together, and then check out all the items with bidder ID 215 together. You cannot combine two constituents/Bidders IDs into one checkout.