This bug exists since 188.8.131.52. A change was made to the inventory's behavior if you close it while an item is in your cursor: you get it back in your inventory rather than dropping it on the ground. I tried pressing both T and ESC at the same time while holding an item... and, as I suspected, I got it back AND dropped it on the ground at the same time, effectively duping it. I found this a long while ago and even reported it, but it got no replies and it wasn't fixed in subsequent updates. I've never seen anyone else talk about it either. Weird, considering this is a very logical and easily-reproduced bug. EDIT: It works with or without autopause, but in the latter case (and multiplayer) it is seemingly frame-perfect. With autopause, as long as you press both buttons in roughly the same time frame, it works.