fix: silently skip duplicate TestFiles.add() instead of raising #287

Job	Run time
check-linked-issue	4s
determine-changes / detect	9s
coverage	7m 40s
type-check	27s
unit-tests (ubuntu-latest, 3.14)	5m 42s
unit-tests (ubuntu-latest, 3.13)	7m 17s
unit-tests (windows-latest, 3.13)	9m 42s
unit-tests (ubuntu-latest, 3.10)	6m 51s
e2e-python (futurehouse-structure, end_to_end_test_futurehouse.py, 5)	8s
prek / prek	8s
unit-tests (ubuntu-latest, 3.11)	7m 7s
e2e-python (tracer-replay, end_to_end_test_tracer_replay.py, 10)	8s
e2e-python (bubble-sort-unittest, end_to_end_test_bubblesort_unittest.py, 40)	9s
e2e-python (topological-sort, end_to_end_test_topological_sort_worktree.py, 5)	8s
e2e-python (coverage-e2e, end_to_end_test_coverage.py, black)	7s
e2e-python (async-optimization, end_to_end_test_async.py, 10)	7s
e2e-python (benchmark-bubble-sort, end_to_end_test_benchmark_sort.py, 5)	10s
unit-tests (ubuntu-latest, 3.12)	7m 8s
e2e-python (init-optimization, end_to_end_test_init_optimization.py, 10)	20s
e2e-python (bubble-sort-pytest-nogit, end_to_end_test_bubblesort_pytest.py, 70, true)	8s
e2e-js	0s
unit-tests (ubuntu-latest, 3.9)	9m 9s
e2e-java	0s
required checks passed	3s
	1h 2m 52s

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