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This is a job for Extract Method. Most of the effort goes in to picking the right places to slice the method in to bits.
It's easier to know where to slice when you have a second (or 3rd!) example - it's easier to see the duplication. On the other hand, refactoring 3 is more work than refactoring 1. I took a quick stab at writing a new test, saw the duplication, deleted it, and then refactored. Call this an educated guess.
Here's what I ended up with:
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I'm curious about this new assert. Some TDD gurus mention custom asserts as one element of advanced TDD, so I am going to pay attention here and see if anything interesting happens.
Next up, a new feature, with a new test. Let's see if my refactoring works out.
*Arlo Belshee goes a step further: don't worry about duplication between tests as much as making the rests read cleanly. They should say exactly what you want to test, no more, no less. He says tests should be "WET" - Write Explicit Tests, while product code should be "DRY" - Don't Repeat Yourself.