A good abstraction? As in one? I'd go so far as to say the process of discovering and refining abstractions is the most important part of software engineering. A large project has dozens of abstractions, and some of them are "wrong" at any time, as you discover over time. None are ever perfect. If you wait to stop duplicating code until you have the "right" abstraction, you are just putting off the hard part of developing software and taking on tech debt.

Half of your abstractions are wrong. The hard part is knowing which half.

I once worked at a place with abstractions I found to be beautifully perfect. The people that wrote the base framework had done similar things two or so times previously and got it right the third time. You couldn’t write slow or hard to operate code there without really trying hard.

Good abstraction does one single thing and does it well. Bad abstraction starts from the premise of becoming a dumping ground. If that is the case, the best and ideal scenario is splitting the abstraction into many ones to make the job better.