diff --git a/src/evaluate.ts b/src/evaluate.ts
index 93a77fc..be171df 100644
--- a/src/evaluate.ts
+++ b/src/evaluate.ts
@@ -5,9 +5,9 @@ import { aggregate } from './aggregate.js'
 import { report } from './usage.js'
 
 /**
- * Sweeps `config.models`, runs the task on each case, scores the output, and
- * aggregates per model. A case whose task throws is recorded with `output: null`
- * and does not abort the run.
+ * Sweeps `config.models` (deduplicated), runs the task on each case, scores the
+ * output, and aggregates per model. A case whose task throws is recorded with
+ * `output: null` and the error message, and does not abort the run.
  *
  * @param name   name for this eval, included in the report.
  * @param config the eval configuration.
@@ -17,8 +17,9 @@ export async function evaluate<I, O, E>(name: string, config: EvalConfig<I, O, E
   const cases = await (typeof config.data === 'function' ? config.data() : config.data)
   assert(cases && cases.length > 0, 'data is required')
 
-  const models = config.models
-  assert(models && models.length > 0, 'models is required')
+  assert(config.models && config.models.length > 0, 'models is required')
+  // A repeated model id would re-run every case only to overwrite its own entry.
+  const models = [...new Set(config.models)]
 
   const scorers = config.scorers
   assert(scorers && scorers.length > 0, 'scorers is required')
@@ -62,6 +63,7 @@ export async function evaluate<I, O, E>(name: string, config: EvalConfig<I, O, E
           score: 0,
           scores: [],
           output: null,
+          error: error.message,
         })
 
         continue
diff --git a/src/types.ts b/src/types.ts
index 47e15c0..74c6133 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -76,6 +76,8 @@ export type CaseResult<O> = {
   tags: string[];
   /** The task output, or `null` if the task threw. */
   output: O | null;
+  /** The task's error message, present only when the task threw. */
+  error?: string;
   /** Weighted mean over applicable (non-`null`) scorers. */
   score: number;
   /** Per-scorer breakdown. */
diff --git a/test/evaluate.test.ts b/test/evaluate.test.ts
index 90c2af7..b98786b 100644
--- a/test/evaluate.test.ts
+++ b/test/evaluate.test.ts
@@ -98,6 +98,20 @@ describe('evaluate', () => {
     const results = report.byModel.m1?.cases!
     expect(results).toHaveLength(2)
     expect(results.every(c => c.output === null && c.score === 0 && c.scores.length === 0)).toBe(true)
+    expect(results.every(c => c.error === 'task exploded')).toBe(true)
+  })
+
+  it('leaves error unset when the task succeeds', async () => {
+    const report = await evaluate('fine', config({ data: [cases[0]!] }))
+    expect(report.byModel.m1?.cases[0]?.error).toBeUndefined()
+  })
+
+  it('records a non-Error throw as its string form', async () => {
+    const boom: Task<Input, Output> = async () => {
+      throw 'plain string failure'
+    }
+    const report = await evaluate('boom', config({ task: boom, data: [cases[0]!] }))
+    expect(report.byModel.m1?.cases[0]?.error).toBe('plain string failure')
   })
 
   it('records a thrown scorer as score 0 with the error surfaced, without aborting', async () => {
@@ -248,15 +262,15 @@ describe('evaluate', () => {
     expect(report.byModel.m1?.cases[0]?.score).toBe(2)
   })
 
-  it('runs the task once per duplicate model id but keeps only one report entry (silent overwrite + wasted work)', async () => {
+  it('dedupes repeated model ids instead of re-running and overwriting', async () => {
     let calls = 0
     const counting: Task<Input, Output> = async (input, ctx) => {
       calls++
       return { y: input.x * 2, model: ctx.model }
     }
     const report = await evaluate('dup', config({ task: counting, models: ['m1', 'm1'], data: [cases[0]!] }))
-    expect(Object.keys(report.byModel)).toEqual(['m1']) // the second sweep clobbers the first
-    expect(calls).toBe(2) // ...yet the task still ran twice — duplicate ids should arguably dedupe or throw
+    expect(Object.keys(report.byModel)).toEqual(['m1'])
+    expect(calls).toBe(1)
   })
 
   it('rejects a scorer with a non-positive weight', async () => {