Idea for duping

if you mean on one account, that is impossible. if you mean with somehow two same accounts logged in at same
time, then that is possible, but only in theory.

My friend uses his kong account as a storage area for his stuff and his steam account as his main. Thus it is possible.

I believe @atenzor meant having the same account logged in twice, rather than having two different accounts logged in at the same time.

@SlamDrag

first, you said: "What if it were possible to trade with yourself. Now that would be helpful lol."
then, you answer your own question with: "My friend uses his kong account as a storage area
for his stuff and his steam account as his main. Thus it is possible."

Now, from this, we can conclude two things: that yes, indeed, it is POSSIBLE to trade between your "mule" and
your "main" or however you want to interpret two accounts owned by same person, which is normal trading,
which was here for a LONG time in this game, I hope you didn't just discover it NOW?

and the second thing is if you meant, but you didn't complete your sentence in that case, that is it possible
to somehow "dupe" with trading system by itself (between two accounts owned by one person). In which case,
so far, it's not possible.