I get the same item on multiple accounts too, which is really odd.
I see this often. For the most part it seems to me that items drop in groupings. I don't know if the groupings are random, or based on a specific time, etc.
If I get armor, I often get several pieces of armor from the same era / content (salvage, dynamis, etc.). If I get a void/shard I tend to get two. It is extremely hard to quantify, but also pretty difficult to ignore.