In the GeoNames snapshot used for this site, the postal place name Washington appears in 25 states and the District of Columbia, but not in Washington State. That is a finding about this postal dataset, not an exhaustive claim about every settlement. For a WA fixture, this generator uses Seattle; for the capital, use the Washington, DC generator.
That makes the state-versus-district collision a genuine data problem rather than a pedantic one. "Washington" in a city field means DC or a small town in Pennsylvania, Indiana, Missouri or twenty-three other states; "Washington" in a state field means the Pacific Northwest; and "WA" is the state while "DC" is the district. Any importer that resolves a place name to a state, or that treats a city and a state field as interchangeable when one is empty, will scatter records across the country here.
Resolve a US place using its city, state and complete ZIP rather than the name alone. The stored Washington aggregates contain 716 postal records across 14 prefixes and 39 county labels. Those prefixes run from 980 to 994 with gaps. This distinguishes their WA reference context from the capital, but it does not prove an invented street or house exists.
Vancouver is another useful address-test case: Vancouver, Washington and Vancouver, British Columbia belong to different countries. Keep country and state or province explicit, then validate the postal field within that context. The Washington generator on this page stays in Seattle; the Vancouver example illustrates a separate validation case.
Two more notes for a fixture. The largest prefix by code count is 983 around Bremerton rather than anything in Seattle, the usual consequence of territory over population. And because the prefixes are non-contiguous, Washington needs a prefix set rather than a range, unlike neighbouring Oregon, whose ten prefixes run 970 to 979 unbroken.