diff --git a/.claude/rules/prisma-db.md b/.claude/rules/prisma-db.md
index d9a9ac420..0189c0125 100644
--- a/.claude/rules/prisma-db.md
+++ b/.claude/rules/prisma-db.md
@@ -141,6 +141,12 @@ When a migration leaves `finished_at = NULL` in `_prisma_migrations`:
 
 A `--rolled-back` migration is permanently skipped by `migrate deploy`; fixing the original file has no effect.
 
+## Merged Data: Document Uniqueness Loss
+
+Service functions that merge records from multiple sources (e.g., `getMergedTasksMap()` combining base tasks + `ContestTaskPair` entries) lose individual field uniqueness. Document that results contain duplicates in "unique" DB fields.
+
+Example: `getMergedTasksMap()` returns multiple entries with same `task_id` but different `contest_id` — callers must not assume `task_id` is unique. UI layer must use composite keys (see svelte-components.md `{#each}` patterns).
+
 ## Validate Constraints
 
 Prisma does not support `@@check`. To add one:
diff --git a/.claude/rules/svelte-components.md b/.claude/rules/svelte-components.md
index 4569ac508..bcb24da19 100644
--- a/.claude/rules/svelte-components.md
+++ b/.claude/rules/svelte-components.md
@@ -37,6 +37,7 @@ Use `$props()`, `$state()`, `$derived()`, `$effect()` in all components:
 ## `{#each}` Patterns
 
 - Always key: `(item.id)` or `(i)`
+- **Key MUST be unique per iteration** — if domain allows duplicates, use composite key (e.g. `contest_id + '-' + task_id`)
 - Filter **before**, not inside with `{#if}`
 - Use `{:else}` for empty lists
 
@@ -48,6 +49,8 @@ Use `$props()`, `$state()`, `$derived()`, `$effect()` in all components:
 {/each}
 ```
 
+**Common Trap:** `task_id` alone is NOT unique when same task appears in multiple contests. Use `contest_id + '-' + task_id` as composite key (see issue #3460 & PR #3442).
+
 ## Snippets vs Components
 
 - **Snippet**: needs `$state` access, pure display, same-file; params require explicit type `{#snippet label(item: Item)}`
diff --git a/src/lib/components/TaskList.svelte b/src/lib/components/TaskList.svelte
index bbfa66303..3cf080f56 100644
--- a/src/lib/components/TaskList.svelte
+++ b/src/lib/components/TaskList.svelte
@@ -97,7 +97,7 @@
         </TableHead>
 
         <TableBody class="divide-y divide-gray-200 dark:divide-gray-700">
-          {#each taskResults as taskResult (taskResult.task_id)}
+          {#each taskResults as taskResult (taskResult.contest_id + '-' + taskResult.task_id)}
             <TableBodyRow
               id={taskResult.contest_id + '-' + taskResult.task_id}
               class={getBackgroundColorFrom(taskResult.status_name)}