diff --git a/core/flamingock-core-commons/src/main/java/io/flamingock/internal/common/core/journal/DuplicateStreamSequenceException.java b/core/flamingock-core-commons/src/main/java/io/flamingock/internal/common/core/journal/DuplicateStreamSequenceException.java
new file mode 100644
index 000000000..7f9c9c024
--- /dev/null
+++ b/core/flamingock-core-commons/src/main/java/io/flamingock/internal/common/core/journal/DuplicateStreamSequenceException.java
@@ -0,0 +1,87 @@
+/*
+ * Copyright 2026 Flamingock (https://www.flamingock.io)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.flamingock.internal.common.core.journal;
+
+import io.flamingock.internal.common.core.error.FlamingockException;
+
+/**
+ * Signals that a journal event could not be appended because its
+ * {@code (streamId, streamSequence)} position was already taken.
+ * <p>
+ * Every audit store already rejects a duplicate position — MongoDB through a unique index on
+ * {@code (streamId, streamSequence)} plus an insert that is never an upsert, SQL through
+ * {@code PRIMARY KEY (stream_id, stream_sequence)} and a plain {@code INSERT}, Couchbase because the
+ * document key <em>is</em> the position and it inserts rather than upserts, and DynamoDB because the pair
+ * is the table's primary key, guarded by a condition expression. What they do not share is <em>how</em>
+ * they say so: {@code DuplicateKeyException}, a {@code SQLException}, {@code DocumentExistsException}, or
+ * a {@code ConditionalCheckFailedException} buried inside a transaction cancellation.
+ * <p>
+ * This is the common type they translate to, so a caller can tell "that position is taken" apart from
+ * "the write failed for some other reason" without knowing which backend it is talking to. That
+ * distinction is what makes a portable retry possible: retrying a genuine infrastructure failure would
+ * be wrong, and retrying nothing at all would leave a recoverable collision as a hard failure.
+ * <p>
+ * Nothing throws this yet. Each store translates its own exception in its own change; see
+ * {@code docs/AUDIT_COMPACTION_CONTRIBUTOR_GUIDE.md} for how the journal work is split.
+ *
+ * <h2>Why it carries the position</h2>
+ * A retry needs to know which position was rejected, and an operator reading a log needs both halves:
+ * {@code streamSequence} on its own identifies nothing, since every stream has its own sequence 1.
+ */
+public class DuplicateStreamSequenceException extends FlamingockException {
+
+    private final String streamId;
+    private final long streamSequence;
+
+    /**
+     * @param streamId       the stream whose position was taken
+     * @param streamSequence the position within that stream
+     * @param cause          the backend's own exception. Worth keeping rather than swallowing: without it,
+     *                       a position collision and a database rejecting the write for an unrelated
+     *                       reason are indistinguishable after the fact
+     */
+    public DuplicateStreamSequenceException(String streamId, long streamSequence, Throwable cause) {
+        super(message(streamId, streamSequence), cause);
+        this.streamId = streamId;
+        this.streamSequence = streamSequence;
+    }
+
+    /**
+     * For the case where a store detects the duplicate without an exception to wrap — for instance from an
+     * affected-row count, or a conditional write that reports failure as a value.
+     *
+     * @param streamId       the stream whose position was taken
+     * @param streamSequence the position within that stream
+     */
+    public DuplicateStreamSequenceException(String streamId, long streamSequence) {
+        super(message(streamId, streamSequence));
+        this.streamId = streamId;
+        this.streamSequence = streamSequence;
+    }
+
+    public String getStreamId() {
+        return streamId;
+    }
+
+    public long getStreamSequence() {
+        return streamSequence;
+    }
+
+    private static String message(String streamId, long streamSequence) {
+        return "Journal event position already taken [streamId=" + streamId
+                + " streamSequence=" + streamSequence + "]";
+    }
+}
diff --git a/core/flamingock-core-commons/src/test/java/io/flamingock/internal/common/core/journal/DuplicateStreamSequenceExceptionTest.java b/core/flamingock-core-commons/src/test/java/io/flamingock/internal/common/core/journal/DuplicateStreamSequenceExceptionTest.java
new file mode 100644
index 000000000..f33f03f0f
--- /dev/null
+++ b/core/flamingock-core-commons/src/test/java/io/flamingock/internal/common/core/journal/DuplicateStreamSequenceExceptionTest.java
@@ -0,0 +1,88 @@
+/*
+ * Copyright 2026 Flamingock (https://www.flamingock.io)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.flamingock.internal.common.core.journal;
+
+import io.flamingock.internal.common.core.error.FlamingockException;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * There is little behaviour in an exception class, so these cover only what something else depends on:
+ * the position the retry reads back, the cause a diagnosis needs, and the message an operator sees.
+ */
+class DuplicateStreamSequenceExceptionTest {
+
+    @Test
+    @DisplayName("the message names both halves of the position")
+    void messageNamesTheWholePosition() {
+        DuplicateStreamSequenceException exception =
+                new DuplicateStreamSequenceException("stage-one", 42L, new IllegalStateException("taken"));
+
+        String message = exception.getMessage();
+        // Both halves, because a sequence on its own identifies nothing: every stream has a position 1.
+        // This message is all an operator gets from a log line.
+        assertTrue(message.contains("stage-one"), message);
+        assertTrue(message.contains("42"), message);
+    }
+
+    @Test
+    @DisplayName("the backend's own exception is preserved as the cause")
+    void preservesTheBackendCause() {
+        IllegalStateException backendFailure = new IllegalStateException("E11000 duplicate key");
+
+        DuplicateStreamSequenceException exception =
+                new DuplicateStreamSequenceException("stage-one", 42L, backendFailure);
+
+        // The one that matters. Swallowing it would make a position collision and a database rejecting
+        // the write for an unrelated reason look identical after the fact.
+        assertSame(backendFailure, exception.getCause());
+    }
+
+    @Test
+    @DisplayName("the position is readable, which is what a retry needs")
+    void exposesThePosition() {
+        DuplicateStreamSequenceException exception =
+                new DuplicateStreamSequenceException("stage-one", 42L, new IllegalStateException());
+
+        assertEquals("stage-one", exception.getStreamId());
+        assertEquals(42L, exception.getStreamSequence());
+    }
+
+    @Test
+    @DisplayName("a store with no exception to wrap still reports the position")
+    void supportsDetectionWithoutABackendException() {
+        // For backends that report a rejected conditional write as a value rather than by throwing.
+        DuplicateStreamSequenceException exception =
+                new DuplicateStreamSequenceException("stage-one", 42L);
+
+        assertNull(exception.getCause());
+        assertEquals("stage-one", exception.getStreamId());
+        assertEquals(42L, exception.getStreamSequence());
+    }
+
+    @Test
+    @DisplayName("it is a FlamingockException, so existing handlers see it")
+    void isAFlamingockException() {
+        // Relevant if it ever escapes un-retried: it surfaces as a Flamingock error rather than a bare
+        // runtime exception.
+        assertTrue(new DuplicateStreamSequenceException("stage-one", 1L) instanceof FlamingockException);
+    }
+}