Duplicate items and respawned blocks

In a world owned by my brother, I was mining in my mine, then suddenly the host of the server (my brother) suddenly disconnected. After entering the world again, I went mining again in the same mine, with the same stuff. The stone in the mine "respawned", so it was present like it was before the last mining session. Soon, I ran into the stuff in my inventory, dropped inside a few stone blocks, so I had double pickaxes, double cobble, and double whatever else I had in my inventory.