diff --git a/CHANGELOG.md b/CHANGELOG.md
index ac2395d2..50d87c8a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,7 @@ Latest
 * [#201](https://github.com/cleverage/process-bundle/issues/201) Fix minor defects: error messages of TransformerTrait, RulesTransformer and ExpressionLanguageMapTransformer, useless `setRequired()` in ImplodeTransformer and SprintfTransformer, stray namespace in TrimTransformer, wrong or missing docblocks. Add tests.
 * [#220](https://github.com/cleverage/process-bundle/issues/220) Fix the stop error strategy: throw a `ProcessFailedException` (with the original exception as `previous`) instead of a `FatalError`, so that the command exits with a non-zero code when a process fails and `ProcessLauncherTask` detects failed sub-processes. Update documentation, add tests.
 * [#221](https://github.com/cleverage/process-bundle/issues/221) Fix CsvSplitterTask: each produced file contains `max_lines` data lines (instead of `max_lines - 2`), no infinite loop with `max_lines` <= 2 (`max_lines` must now be an integer greater than 0), no header-only file at the end. Update documentation, add tests.
+* [#223](https://github.com/cleverage/process-bundle/issues/223) Fix CounterTask: the final count is outputted once, as `flush()` may be called several times. Document that `flush()` implementations must be idempotent. Update documentation, add tests.
 
 ## Deprecated
 * [#189](https://github.com/cleverage/process-bundle/issues/189) EventDispatcherTask: when `event_name` is set, listening to `CleverAge\ProcessBundle\Event\EventDispatcherTaskEvent` is deprecated (the event is still dispatched under its class name, with an `E_USER_DEPRECATED` error, if it has listeners). Listen to the configured `event_name` instead: the BC layer will be removed in v6.0.
diff --git a/docs/02-task_types.md b/docs/02-task_types.md
index 9701f680..fc40b2c4 100644
--- a/docs/02-task_types.md
+++ b/docs/02-task_types.md
@@ -102,8 +102,7 @@ Examples:
 - [SimpleBatchTask](reference/tasks/simple_batch_task.md) groups inputs by batches of `batch_count` elements: each
   full batch is outputted during `execute`, and the last incomplete batch during `flush`.
 - [CounterTask](reference/tasks/counter_task.md) outputs the count every `flush_every` items, and the current count on
-  `flush` (unless it is a multiple of `flush_every`): as `flush` may be called several times, the same count can be
-  outputted more than once.
+  `flush` (unless it is a multiple of `flush_every`, or it was already outputted by a previous `flush`).
 
 ## Initializable tasks
 
diff --git a/docs/03-custom_tasks.md b/docs/03-custom_tasks.md
index 1020c0cd..f0cacf9d 100644
--- a/docs/03-custom_tasks.md
+++ b/docs/03-custom_tasks.md
@@ -173,7 +173,9 @@ memory footprint.
 [notice about blocking tasks](02-task_types.md#blocking-tasks) about memory usage.
 
 Tasks should not be both Iterable and Blocking. If you need to buffer data and output it by chunks, look at
-`CleverAge\ProcessBundle\Model\FlushableTaskInterface` (see [flushable tasks](02-task_types.md#flushable-tasks)).
+`CleverAge\ProcessBundle\Model\FlushableTaskInterface` (see [flushable tasks](02-task_types.md#flushable-tasks)). As
+`flush` may be called several times on the same task, it must be idempotent: once the buffer is flushed, a new call
+must skip the state (`ProcessState::setSkipped(true)`) instead of outputting the same data again.
 
 ## Transformers
 
diff --git a/docs/reference/tasks/counter_task.md b/docs/reference/tasks/counter_task.md
index 9cc9a5af..79067612 100644
--- a/docs/reference/tasks/counter_task.md
+++ b/docs/reference/tasks/counter_task.md
@@ -53,5 +53,4 @@ Notes
 
 * `flush()` can be called several times during a process (see
   [Advanced workflow](../../04-advanced_workflow.md)), e.g. once at the end of the upstream iteration and once when
-  the counter itself is resolved. Each call outputs the current count again (unless it is a multiple of `flush_every`),
-  so the final count may be sent more than once to the next tasks.
+  the counter itself is resolved. The final count is only outputted by the first call; the next ones are skipped.
diff --git a/src/Model/FlushableTaskInterface.php b/src/Model/FlushableTaskInterface.php
index 8f56f5fa..1bb4cadc 100644
--- a/src/Model/FlushableTaskInterface.php
+++ b/src/Model/FlushableTaskInterface.php
@@ -15,6 +15,10 @@
 
 /**
  * When iterations are over, this allows task that have some inner buffer to flush it to the output.
+ *
+ * flush() may be called several times on the same task during a process (once for each resolved ancestor, and each time
+ * an upstream iterable task finishes its iterations): implementations must be idempotent, and skip the state
+ * (ProcessState::setSkipped(true)) when there is nothing new to output.
  */
 interface FlushableTaskInterface extends TaskInterface
 {
diff --git a/src/Task/CounterTask.php b/src/Task/CounterTask.php
index 9b2020e5..12e2a650 100644
--- a/src/Task/CounterTask.php
+++ b/src/Task/CounterTask.php
@@ -26,6 +26,11 @@ class CounterTask extends AbstractConfigurableTask implements FlushableTaskInter
 {
     protected int $counter = 0;
 
+    /**
+     * Count already outputted by a flush, as flush() may be called several times.
+     */
+    protected ?int $flushedCounter = null;
+
     public function execute(ProcessState $state): void
     {
         ++$this->counter;
@@ -43,11 +48,14 @@ public function execute(ProcessState $state): void
     public function flush(ProcessState $state): void
     {
         $modulo = $this->getOption($state, 'flush_every');
-        if (0 === $this->counter % $modulo) {
+        if (0 === $this->counter % $modulo || $this->counter === $this->flushedCounter) {
             $state->setSkipped(true);
-        } else {
-            $state->setOutput($this->counter);
+
+            return;
         }
+
+        $this->flushedCounter = $this->counter;
+        $state->setOutput($this->counter);
     }
 
     protected function configureOptions(OptionsResolver $resolver): void
diff --git a/tests/Task/CounterTaskTest.php b/tests/Task/CounterTaskTest.php
new file mode 100644
index 00000000..7e71d283
--- /dev/null
+++ b/tests/Task/CounterTaskTest.php
@@ -0,0 +1,118 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of the CleverAge/ProcessBundle package.
+ *
+ * Copyright (c) Clever-Age
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace CleverAge\ProcessBundle\Tests\Task;
+
+use CleverAge\ProcessBundle\Configuration\ProcessConfiguration;
+use CleverAge\ProcessBundle\Configuration\TaskConfiguration;
+use CleverAge\ProcessBundle\Context\ContextualOptionResolver;
+use CleverAge\ProcessBundle\Model\AbstractConfigurableTask;
+use CleverAge\ProcessBundle\Model\ProcessHistory;
+use CleverAge\ProcessBundle\Model\ProcessState;
+use CleverAge\ProcessBundle\Task\CounterTask;
+use PHPUnit\Framework\TestCase;
+
+#[\PHPUnit\Framework\Attributes\CoversClass(CounterTask::class)]
+#[\PHPUnit\Framework\Attributes\UsesClass(AbstractConfigurableTask::class)]
+#[\PHPUnit\Framework\Attributes\UsesClass(ProcessConfiguration::class)]
+#[\PHPUnit\Framework\Attributes\UsesClass(TaskConfiguration::class)]
+#[\PHPUnit\Framework\Attributes\UsesClass(ContextualOptionResolver::class)]
+#[\PHPUnit\Framework\Attributes\UsesClass(ProcessHistory::class)]
+#[\PHPUnit\Framework\Attributes\UsesClass(ProcessState::class)]
+class CounterTaskTest extends TestCase
+{
+    public function testOutputsTheCountEveryFlushEvery(): void
+    {
+        [$task, $state] = $this->createTask(2);
+
+        self::assertSame([null, 2, null, 4, null], $this->executeAll($task, $state, 5));
+    }
+
+    public function testFlushOutputsTheFinalCountOnlyOnce(): void
+    {
+        [$task, $state] = $this->createTask(2);
+        $this->executeAll($task, $state, 5);
+
+        self::assertSame(5, $this->flush($task, $state));
+        self::assertNull($this->flush($task, $state));
+        self::assertNull($this->flush($task, $state));
+    }
+
+    public function testFlushSkipsWhenTheCountWasAlreadyOutputted(): void
+    {
+        [$task, $state] = $this->createTask(2);
+        $this->executeAll($task, $state, 4);
+
+        self::assertNull($this->flush($task, $state));
+    }
+
+    public function testFlushSkipsWithoutExecution(): void
+    {
+        [$task, $state] = $this->createTask(2);
+
+        self::assertNull($this->flush($task, $state));
+    }
+
+    public function testFlushOutputsANewCountAfterNewExecutions(): void
+    {
+        [$task, $state] = $this->createTask(3);
+        $this->executeAll($task, $state, 2);
+        self::assertSame(2, $this->flush($task, $state));
+
+        $this->executeAll($task, $state, 2);
+        self::assertSame(4, $this->flush($task, $state));
+        self::assertNull($this->flush($task, $state));
+    }
+
+    /**
+     * @return array{CounterTask, ProcessState}
+     */
+    private function createTask(int $flushEvery): array
+    {
+        $processConfiguration = new ProcessConfiguration('test', []);
+        $state = new ProcessState($processConfiguration, new ProcessHistory($processConfiguration));
+        $state->setContextualOptionResolver(new ContextualOptionResolver());
+        $state->setContext([]);
+        $state->setTaskConfiguration(new TaskConfiguration('counter', CounterTask::class, ['flush_every' => $flushEvery]));
+        $task = new CounterTask();
+        $task->initialize($state);
+
+        return [$task, $state];
+    }
+
+    /**
+     * Execute the task $count times, collecting outputs (null when skipped).
+     *
+     * @return list<mixed>
+     */
+    private function executeAll(CounterTask $task, ProcessState $state, int $count): array
+    {
+        $outputs = [];
+        for ($i = 0; $i < $count; ++$i) {
+            $state->reset(false);
+            $state->setInput($i);
+            $task->execute($state);
+            $outputs[] = $state->isSkipped() ? null : $state->getOutput();
+        }
+
+        return $outputs;
+    }
+
+    private function flush(CounterTask $task, ProcessState $state): mixed
+    {
+        $state->reset(true);
+        $task->flush($state);
+
+        return $state->isSkipped() ? null : $state->getOutput();
+    }
+}