The deliberate manual POST /payments had no dedupe guard (double-submit
made a second payment), but silently deduping it like the auto paths
would lose a legitimately-identical same-day payment — a money bug. So
the server returns 409 DUPLICATE_SUSPECTED (existing payment attached),
and a shared client helper (client/lib/paymentActions.ts, used by the
tracker inline cell, partial-payment dialog, and bill modal) turns it
into an 'add anyway?' toast that resends with allow_duplicate. Automated
one-click paths keep deduping silently (a repeat there is a retry).
Dropped a now-unused api import in PaymentLedgerDialog (Track E sweep).
Server suite 188 green; typecheck/lint/build clean; probe 17/17.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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