I say 1 or 3.
Student or Learner
Do we say
1 One way to solve the problem is to do something,
2 One way to solve the problem is do something,
3 One way to solve the problem is doing something?
I have seen something like 'All I want to do is + bare infinitive'.
I am not a teacher.
BTW, I would like to know that if someone said to you "all you did was duplicate my effort," would you consider the speaker is a little unfriendly to you?
Not necessarily. If they were right, it wouldn't be unfriendly. It might not be polite but sometimes the truth hurts.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.