What Snook said is pretty much correct. Just want to expand on it a bit here with the rules source.
If you check out the duel rules
here, you can see that point number 3 says:
3. Duels should be of a competitive nature, based in skill and not luck. Duels such as "1 match of wtf.tbm" will not be supported. Any mod is allowed, but 3 games is recommended.
These are the rules/guidelines of duels and it's what we recommend you should follow or at least base your dueling practices on. That said, the duel is between the two parties dueling and if they both agree and are happy with what's going on then whatever, it's your business. As it says in the quote though, if something happens based on the non competitiveness of the duel that you were aware of but now don't like or agree with for whatever reason then we won't help you because it was your own fault. The key concepts here are awareness and acceptance. If you trick someone into a shitty dueling mode and they don't know or agree etc etc it's still against the rules.