There are two situations when there can be two (or more) bids in a row from the same bidder:
Bids below the reserve
If an item has a reserve price, the bids need to reach a certain hidden limit or the seller will not agree to sell the item.
When the bidder sees that their bid was below this reserve, they may try again, until they reach the limit.
Auto bids
If a bidder places a high max bid, we will bid automatically on their behalf when other bids are placed.
If the new bid is higher than the max bid placed by the first bidder, we will start by showing the first bidders max bid and then place a second, higher bid for the new bidder, at the next step on the bidding ladder.
This way, the leading bid will always be the lowest possible amount that is higher than the second highest bid.