If you bid manually as well as the Gixen bid you are bidding against yourself and may well pay far more than necessary for the item. As an example...if the bid is at $100 and you and Gixen bid around $150  then you  will pay that much instead of $102.50. Do you really want to risk it?

It doesn't work like that. You cannot win it twice! And the price you pay is ENTIRELY based on eBay's Bid Increment system. So, you pay the price of the 2nd highest opposition bidder, plus one Bid Increment.

For example, if the other person bids $90...

a) you have a Gixen set at $100, then you win it at $95

b) you have a Gixen set at $100, and then you also manually bid $150 (either before OR after Gixen), then you win it at $95

c) a) you have a Gixen set at $100, and you manually bid $90, then you win it at $95

No matter what you do, there is no possibility to "bid against yourself" on eBay - it's all based on your UserID. The only way that can happen is if you use a different ID, because then you really ARE bidding against yourself.