I've had dupes - not on intent, it just happened on it's own.
It all started like this: one day I logged off with 9 ambers, 9 citrines and and 9 saphires in warehouse (got them from drops). The next day I tried to get the rest to make refineds - finally I bought 1 of each from peddlers for EPs. And the shit was when I checked my warehouse - all gems I found the day before were missing. I complained on eo forum, but they don't give a shit for stuff like that. So I started to keep all valuable items in alt. backpacks.
The follow up came after a few days - I had 5 reds in WH, don't remember what I did, but the next day there were 7. I used 3 to make socket on sword, bought 2 and put them back in WH (so, should've 6). The next day - again 7 reds. It wasn't my intent to dupe anything - it just happened on it's own.
I haven't had any issues with items and euds in my backpack (except that they are removed from hot-keys at random, also have had some eggs missing from WH hatchers).
So, my bet - taking into account number of players, there is mirrored database cluster with load balancing behind. When you log off, you may get different cluster node at next logon.
So, if something crashes or lags, it's possible that you get warehouse data from different cluster node.
If anyone willing to exploit this, the best bet would be - log on 2 accounts. Take items from one warehouse, trade to alt account (leave in backpack), then make both clients to crash (kill soul.exe process, not task from task manager - this is the same as getting DCed or network down), wait some time (like - 30 min.) / or, don't wait but log on immediately, log back and see if you're lucky.