Two engineers can spend a whole afternoon on a failing build, reading the app code, checking the database, and blaming the server, when the real cause is two tests that both created a customer named "Test User".
This is one of the most confusing bugs you will meet, so let's walk through it slowly.
You have two tests. Both create a contact named "Dana Fox".
Run the first one alone: it passes. Run the second one alone: it passes. Run them together: the second one fails. It expected to find one Dana Fox and it found two.