diff --git a/app/models/formsemestre.py b/app/models/formsemestre.py
index 89468ab2f5..93b781b7d4 100644
--- a/app/models/formsemestre.py
+++ b/app/models/formsemestre.py
@@ -273,6 +273,7 @@ class NotesModuleImplInscription(db.Model):
     """Inscription à un module  (etudiants,moduleimpl)"""
 
     __tablename__ = "notes_moduleimpl_inscription"
+    __table_args__ = (db.UniqueConstraint("moduleimpl_id", "etudid"),)
 
     id = db.Column(db.Integer, primary_key=True)
     moduleimpl_inscription_id = db.synonym("id")
diff --git a/migrations/versions/d74b4e16fb3c_scodoc_9_0_51_add_unicity_constraint_on_.py b/migrations/versions/d74b4e16fb3c_scodoc_9_0_51_add_unicity_constraint_on_.py
new file mode 100644
index 0000000000..1c9da8a4e6
--- /dev/null
+++ b/migrations/versions/d74b4e16fb3c_scodoc_9_0_51_add_unicity_constraint_on_.py
@@ -0,0 +1,46 @@
+"""ScoDoc 9.0.51: add unicity constraint on notes_moduleimpl_inscription
+
+Revision ID: d74b4e16fb3c
+Revises: f86c013c9fbd
+Create Date: 2021-10-09 20:08:50.927330
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.orm import sessionmaker  # added by ev
+
+
+# revision identifiers, used by Alembic.
+revision = "d74b4e16fb3c"
+down_revision = "f86c013c9fbd"
+branch_labels = None
+depends_on = None
+
+Session = sessionmaker()
+
+
+def upgrade():
+    # Added by ev: remove duplicates
+    bind = op.get_bind()
+    session = Session(bind=bind)
+    session.execute(
+        """
+    DELETE FROM notes_moduleimpl_inscription i1
+    USING notes_moduleimpl_inscription i2
+    WHERE i1.id < i2.id
+    AND i1.moduleimpl_id = i2.moduleimpl_id
+    AND i1.etudid = i2.etudid;
+    """
+    )
+
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_unique_constraint(
+        None, "notes_moduleimpl_inscription", ["moduleimpl_id", "etudid"]
+    )
+    # ### end Alembic commands ###
+
+
+def downgrade():
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_constraint(None, "notes_moduleimpl_inscription", type_="unique")
+    # ### end Alembic commands ###