diff --git a/src/routes/main_routes.py b/src/routes/main_routes.py
index 3b669195..dcf46d9c 100644
--- a/src/routes/main_routes.py
+++ b/src/routes/main_routes.py
@@ -23,7 +23,7 @@
     SkillDifficulty,
     validate_skill_progression,
 )
-from utils.code_review import CodeReviewManager
+from utils.code_review import CodeReviewManager, SubmissionAlreadyExistsError
 from config import Config
 from utils.portfolio_analyzer import analyze_portfolio
 import math
@@ -706,6 +706,8 @@ def submit_code_for_review():
             "success": True,
             "submission": submission
         }), 201
+    except SubmissionAlreadyExistsError as e:
+        return jsonify({"error": str(e)}), 409
     except Exception as e:
         return jsonify({"error": str(e)}), 400
 
diff --git a/src/utils/code_review.py b/src/utils/code_review.py
index ad4d3f7d..08fbdc59 100644
--- a/src/utils/code_review.py
+++ b/src/utils/code_review.py
@@ -10,6 +10,10 @@
 from datetime import datetime, timezone
 
 
+class SubmissionAlreadyExistsError(Exception):
+    """Raised when a code submission re-uses an existing submission_id."""
+
+
 class ReviewStatus(Enum):
     """Status of a code review."""
     PENDING = "pending"
@@ -106,6 +110,11 @@ def submit_code(
             "metrics": {},
         }
 
+        if submission_id in self.submissions:
+            raise SubmissionAlreadyExistsError(
+                f"Submission {submission_id} already exists"
+            )
+
         self.submissions[submission_id] = submission
         return submission
 
diff --git a/tests/test_code_review.py b/tests/test_code_review.py
index f8e6f720..ed891bf2 100644
--- a/tests/test_code_review.py
+++ b/tests/test_code_review.py
@@ -8,6 +8,7 @@
     ReviewStatus,
     CodeQualityCategory,
     FEEDBACK_TEMPLATES,
+    SubmissionAlreadyExistsError,
 )
 
 
@@ -37,6 +38,66 @@ def test_submit_code(self, review_manager):
         assert submission["review_status"] == ReviewStatus.PENDING.value
         assert submission["submitted_at"] is not None
 
+    def test_submit_code_rejects_duplicate_id(self, review_manager):
+        """Re-submitting an existing submission_id must be rejected (issue #1871)."""
+        review_manager.submit_code(
+            submission_id="sub_001",
+            user_id="user_123",
+            project_id=1,
+            code="print('hello')",
+            language="python",
+        )
+
+        with pytest.raises(SubmissionAlreadyExistsError):
+            review_manager.submit_code(
+                submission_id="sub_001",
+                user_id="user_456",
+                project_id=2,
+                code="print('overwrite')",
+                language="python",
+            )
+
+    def test_resubmission_does_not_clobber_review_progress(self, review_manager):
+        """A rejected re-submit must leave review_count and metrics intact (issue #1871)."""
+        review_manager.submit_code(
+            submission_id="sub_001",
+            user_id="user_123",
+            project_id=1,
+            code="print('hello')",
+            language="python",
+        )
+        review = review_manager.start_review(
+            submission_id="sub_001",
+            reviewer_id="reviewer_001",
+        )
+        review_manager.score_category(
+            review_id=review["review_id"],
+            category="functionality",
+            score=90,
+        )
+        review_manager.complete_review(
+            review_id=review["review_id"],
+            summary="Good work",
+        )
+
+        before = review_manager.get_submission("sub_001")
+        assert before["review_count"] == 1
+        assert before["metrics"]["overall_score"] == 90.0
+
+        with pytest.raises(SubmissionAlreadyExistsError):
+            review_manager.submit_code(
+                submission_id="sub_001",
+                user_id="user_123",
+                project_id=1,
+                code="print('overwrite')",
+                language="python",
+            )
+
+        after = review_manager.get_submission("sub_001")
+        assert after["review_count"] == before["review_count"]
+        assert after["metrics"] == before["metrics"]
+        assert after["code"] == "print('hello')"
+
     def test_get_submission(self, review_manager):
         """Test retrieving a submission."""
         review_manager.submit_code(