Excuse me, I have a question that has been bothering me since last night. I don't know where this thought came from
What happens if I distrust the default trust?
Yes, nothing, you just distrust it.
Can I add the same person to the untrusted list? does it have any effect?
It's still has the same effect, distrusting a user means you distrust him/er regardless how many times you put the same username with a tilde.