To minimize the impact on the rest of the code, i kept the aliases as they are (non-flat, nested).
So the fix is only on the flags.string. The fix is future proof if one day aliases become flat.

I prefer expanding the tests to cover this combination rather than adding a specific regression test for the failure. So the test would be named like "opt.string works with multiple aliases" rather than "Issue #9 fixed".

But wait for a second opinion from @ljharb before changing.