If you have 2 minecraft accounts you can dupe your whole inventory. If you enter your single player world on one account, log off, then enter on another account, open it to lan, and log on with the previous account, both accounts will have the same inventory, thus, duping a large amount of items.
MC-70930
Dupe Glitch in minecraft 1.8
Resolved
Duplicate
0
Shweyin Than
(Unassigned)
Unconfirmed
Minecraft 1.8
Retrieved
There are at least a half dozen bug reports on this issue, going back at least to v1.4.6. All of them have been marked "Duplicate" except MC-5405, which has been marked "Invalid". The net result is that the moderators keep saying this is a non-issue, when in fact this continues to be a debilitating issue years after it was initially reported. The bug prohibits multiple players from playing on a single computer. It is open and it needs to be fixed.