Duplicate listing on eBay is a big no-no – eBay considers this spamming and punishes the offenders. I had this punishment given to me recently, and it really changed my style. I had actually done it several times and they just pulled the duplicate listing. They may have even slapped my hand one time, and then pulled it when I called. Then they actually suspended me as well. Wow. I called member services, and they did not allow me back on. They said the next time would be 14 days. Mine were honest mistakes as well, not realizing that I had an item on GTC as well as 30 days, or auction. Sometimes you will be stopped automatically from the relist but sometimes not. I sound like a super offender but I was not: eBay just really hates this particular offense.
I use the tools to determine double listing , as well as taking every re-list that comes up in my unsolds to active to research before I relist it. There I search for keywords not just the title. Even after all that I have found one double listing when we did inventory.
Sometimes you might have a feeling that the business world is stacked against you, that it is hard to achieve business goals like keeping eBay Top Rated Seller because of shipping failures outside of your control for example. But I have found the more systeamtic you are, the easier it is to maintain order.
Inventory is difficult, or at least time consuming and it is my least favorite part of the business. (Collecting the money is my favorite, followed closely with the friendships I have developed.) I am currently converting everything in my store to GTC with a few auctions to bring more buzz to the store. The main thing that brings the most sales on is consistently listing every day – and you will be able to do that when you are sure that you have not double listed.