To that end, I think a better guideline than "only have one assertion per test" would be "only test one behavior per test". So if you're writing a test for appending an element to a vector, it's probably fine to assert that the size increased by one AND assert that the last element in the vector is now the element that you inserted.
The thing I see people do that's more problematic is to basically pile up assertions in a single test, so that the inputs and outputs for the behavior become unclear, and you have to keep track of the intermediate state of the object being tested in your head (assuming they're testing an object). For instance, they might use the same vector, which starts out empty, test that it's empty; then add an element, then test that its size is one; then remove the element, test that the size is 0 again; then resize it, etc. I think that's the kind of testing that the "one assertion per test" rule was designed to target.
With a vector it's easy enough to track what's going on, but it's much harder to see what the discrete behaviors being tested are. With a more complex object, tracking the internal state as the tests go along can be way more difficult. It's a lot better IMO to have a bunch of different tests with clear names for what they're testing that properly set up the state in a way that's explicit. It's then easier to satisfy the above two requirements I listed.
I want to be able to look at a test and know exactly what I don't mind if a little bit of code is repeated - you can make functions if you need to help with the test set up and tear down.