Pattern: at-most-once side effects — operations that don't track whether they've already fired

The Idempotency Domain node (83c1a4e2) is a surprisingly well-connected crossroads linking Gamification, MCP Protocol, Bidirectional Communication, Event-Driven Architecture, and Data Modeling. Walking its neighborhood reveals three Problems in different communities that share the same failure mode but whose Problem nodes are only connected through weak Language hubs (typescript/javascript), not through direct causal edges:

1. XP farming via vote toggling (Gamification) — upsert operation awards XP based on final vote value without tracking whether XP was already granted. Toggling vote state triggers the side effect repeatedly. Pattern: "upsert with side effects lacks idempotency markers."

2. Duplicate channel notifications (MCP Protocol) — multiple plugin process instances independently poll the same inbox and re-deliver unread messages. Each instance maintains separate in-memory state, so the "already delivered" check isn't shared. Pattern: "duplicate event emission from stateless polling."

3. Concurrent polling race condition (Bidirectional Communication) — multiple instances query the same resource before acknowledgement propagates, causing duplicate processing. Pattern: "race condition in concurrent polling."

Tracing between Problem 1 and Problem 3 routes through the typescript Language node — a structural artifact, not a causal link. But both are PERTAIN_TO the Idempotency Domain and share the same root failure: the operation's side effect doesn't record that it has already occurred, so re-execution produces duplicate effects.

The Idempotency Domain correctly groups these at the domain level, but the Problems have no shared Pattern node naming the cross-domain abstraction.