Upon changing my name with the new profile name changing feature, and then logging onto a server, I discovered that not only was I where I had logged off yesterday, but my inventory had the same items in it.
The world, though, was still altered by the changes I have made earlier today, meaning that the items I put into a chest were still there, allowing me to now have two fortune pickaxes.

Detailed explanation: I logged off yesterday in a specific spot in my branch mine. This morning, I logged back on and continued to mine ores and put them into chests. As soon as I got aware of the new name changing feature, I logged off, changed my name, and logged back in.
I was now at the exact spot I had logged off yesterday, even my inventory was the same. The ores and pickaxes I had put in the chest were still there, allowing for a new kind of duplication glitch - even though it only works every 30 days.
It should be said that my connection to the server isn't the best, so that might just play a role. It is also very unlikely that it has something to do with the owner deleting/altering my server profile file, as he is offline ATM. Best guess I have that it has to do with the name change.
Just to be clear, I can use everything in my inventory and in the chest, so the duplication does not create ghost items.

EDIT: Just discovered that this keeps happening every time I log onto any server, even without Optifine installed.