Is duplicated code more tolerable in unit tests?

Readability is more important for tests. If a test fails, you want the problem to be obvious. The developer shouldn't have to wade through a lot of heavily factored test code to determine exactly what failed. You don't want your test code to become so complex that you need to write unit-test-tests.

However, eliminating duplication is usually a good thing, as long as it doesn't obscure anything, and eliminating the duplication in your tests may lead to a better API. Just make sure you don't go past the point of diminishing returns.

Duplicated code is a smell in unit test code just as much as in other code. If you have duplicated code in tests, it makes it harder to refactor the implementation code because you have a disproportionate number of tests to update. Tests should help you refactor with confidence, rather than be a large burden that impedes your work on the code being tested.

If the duplication is in fixture set up, consider making more use of the setUp method or providing more (or more flexible) Creation Methods.

If the duplication is in the code manipulating the SUT, then ask yourself why multiple so-called “unit” tests are exercising the exact same functionality.

If the duplication is in the assertions, then perhaps you need some Custom Assertions. For example, if multiple tests have a string of assertions like:

assertEqual('Joe', person.getFirstName())
assertEqual('Bloggs', person.getLastName())
assertEqual(23, person.getAge())

Then perhaps you need a single assertPersonEqual method, so that you can write assertPersonEqual(Person('Joe', 'Bloggs', 23), person). (Or perhaps you simply need to overload the equality operator on Person.)

As you mention, it is important for test code to be readable. In particular, it is important that the intent of a test is clear. I find that if many tests look mostly the same, (e.g. three-quarters of the lines the same or virtually the same) it is hard to spot and recognise the significant differences without carefully reading and comparing them. So I find that refactoring to remove duplication helps readability, because every line of every test method is directly relevant to the purpose of the test. That's much more helpful for the reader than a random combination of lines that are directly relevant, and lines that are just boilerplate.

That said, sometimes tests are exercising complex situations that are similiar but still significantly different, and it is hard to find a good way to reduce the duplication. Use common sense: if you feel the tests are readable and make their intent clear, and you're comfortable with perhaps needing to update more than a theoretically minimal number of tests when refactoring the code invoked by the tests, then accept the imperfection and move on to something more productive. You can always come back and refactor the tests later, when inspiration strikes!

Implementation code and tests are different animals and factoring rules apply differently to them.

Duplicated code or structure is always a smell in implementation code. When you start having boilerplate in implementation, you need to revise your abstractions.

On the other hand, testing code must maintain a level of duplication. Duplication in test code achieves two goals:

• Keeping tests decoupled. Excessive test coupling can make it hard to change a single failing test that needs updating because the contract has changed.
• Keeping the tests meaningful in isolation. When a single test is failing, it must be reasonably straightforward to find out exactly what it is testing.

I tend to ignore trivial duplication in test code as long as each test method stays shorter than about 20 lines. I like when the setup-run-verify rhythm is apparent in test methods.

When duplication creeps up in the "verify" part of tests, it is often beneficial to define custom assertion methods. Of course, those methods must still test a clearly identified relation that can be made apparent in the method name: assertPegFitsInHole -> good, assertPegIsGood -> bad.

When test methods grow long and repetitive I sometimes find it useful to define fill-in-the-blanks test templates that take a few parameters. Then the actual test methods are reduced to a call to the template method with the appropriate parameters.

As for a lot of things in programming and testing, there is no clear-cut answer. You need to develop a taste, and the best way to do so is to make mistakes.

I agree. The trade off exists but is different in different places.

I'm more likely to refactor duplicated code for setting up state. But less likely to refactor the part of the test that actually exercises the code. That said, if exercising the code always takes several lines of code then I might think that is a smell and refactor the actual code under test. And that will improve readability and maintainability of both the code and the tests.