Can I Accidentally Bid Against Myself?

Yes.   If you were already the high bidder, eBay's proxy bidding system does not raise the price any more. However, if a bidder comes in between, the cost will increase up to the second-highest bid plus the bid increment.

You can bid against yourself only if another bidder comes in between you and your next bid.

If you insist on placing a manual bid when you also have a snipe set for the same auction, make sure that you manually "snipe" close to the end about 10-15 seconds prior to closing, as opposed to several days before. This will minimize any pricing escalation.

Helpful Tip: For auctions, eBay always calculates the price by taking the second-highest bid and adding a bid increment.

Also make sure you understand why timing matters in What Snipe Time Should I Choose?.