1.) Glitch through walls:

First method is to run or rather, "jog", towards any door, fence, building or whatever else you'd like to have access to or inside of.
Keep holding W down until the end, walk or whatever else, jog or run towards a fence, door, house, press escape and log out, log back in to any server and you'll be moved approximately 5 - 10m in the direction you were running.

Second method
The second method is a little bit more finicky to get working, it usually requires good timing.

You can get through almost anything, especially walls and doors using this.
Run towards what you want to get through, and just before you hit whatever it is, jump.
It should now have moved you about 5m or around that area to make up for the jump, as if you were still running.

Third method
It's a very blunt version of the above, just keep running towards what you want to get through, you'll eventually get through.
It works about 40% of the time, if you're lucky or you have a weird connection, you'll have more along the lines of 75%.

2.) Duplicating

Inventory dupe
Word of advice, server jumping and logging in and out excessively might get your character wiped.
But it's solvable by going back to the last server you played on when you had stuff and stay there for a while, then log out and NOT switch servers.

We've all probably heard of this one quite a few times now.
While you're logging in, wait for the screen to say "Ready!" down in the left hand side corner of the screen, count to one and the press exit a couple of times. Not too many, you'll get transferred back to the main menu otherwise and you'll lose the time you need to get back in for your extra log in.
Log back in to the same server you just did it to, it's a wise idea to have it as a favorite to locate it faster, but other than that, you're done.

If it doesn't work, you've done it wrong, keep trying, it's complicated.

Backpack dupe
You need a vehicle (Yes I'm aware that they aren't fully in yet) or other sorts of containers where you could transfer your backpack.
Pull it out and keep holding it over the slots as if you were moving it, don't drop it.
Disconnect, pulling the cable is probably the easiest way o