This next bid often comes as the auctioneer is literally selling out to the last high bidder. The answer is, it depends. It's the auctioneer's call, and he can go either way. Some auctioneers see this situation as one that requires them to reopen the bidding in an effort to be fair and to realize the most money for the lot. Other auctioneers, however, are adamantly opposed to reopening the bidding, and they assert it only slows down the auction, causes the earlier bidder to become disgruntled, and encourages all bidders to sit on their bids until the last possible moment.
Simultaneous bids are one thing, but late bids are quite another.
The auctioneer has no authority to reopen bidding on a lot that she has sold just because a late bid is made. This is because a contract of sale has already been formed with the earlier high bidder, and a late bid gives the auctioneer no authority to unilaterally void this contract and reopen the lot for bidding. Nevertheless, it is not uncommon to see auctioneers reopen bidding for late bids. Why this provision is so misunderstood, I can't explain, but it is. That, however, doesn't make the practice of reopening bidding for a late bid right, and the auctioneer who does it is breaching the contract of sale and setting himself up for a claim and potential legal liability.