Unless it's changed very recently, I thought the rule with duplicate auctions is that only one listing will show but when it gets a bid the next one will show, and when that one gets a bid the third one will show, and so on.  As each one gets a bid the next one will show, but I always took this to mean that all the ones with bids should be showing.

I don't really trust the system to work properly so I avoid having different quantities of the same item wherever possible.  I've had a theory for a while that this is why sellers with the same items on different IDs appear to be throttled.  I've noticed that quite a few of the sellers who talk about being throttled say they sell the same items on both IDs, just in different quantities.  I also don't sell the same types of item on different IDs for this reason.