diff --git a/alembic/versions/03d4f000f445_add_unique_constraint_for_company_.py b/alembic/versions/03d4f000f445_add_unique_constraint_for_company_.py
new file mode 100644
index 0000000..b88c4e6
--- /dev/null
+++ b/alembic/versions/03d4f000f445_add_unique_constraint_for_company_.py
@@ -0,0 +1,33 @@
+"""add unique constraint for company person linking table
+
+Revision ID: 03d4f000f445
+Revises: 85577ce9f2ab
+Create Date: 2026-07-22 09:17:46.576581
+
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "03d4f000f445"
+down_revision: Union[str, Sequence[str], None] = "85577ce9f2ab"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table("zuordnung_personen_unternehmen") as batch_op:
+        batch_op.create_unique_constraint("uq_un_id_pers_id", ["un_id", "pers_id"])
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table("zuordnung_personen_unternehmen") as batch_op:
+        batch_op.drop_constraint("uq_un_id_pers_id")
+    # ### end Alembic commands ###
diff --git a/src/wce_crm/db.py b/src/wce_crm/db.py
index 737ed5f..16350dc 100644
--- a/src/wce_crm/db.py
+++ b/src/wce_crm/db.py
@@ -7,7 +7,7 @@ from pathlib import Path
 
 import polars as pl
 import sqlalchemy as sql
-from sqlalchemy import Column, String, Table, TypeDecorator, event
+from sqlalchemy import Column, String, Table, TypeDecorator, UniqueConstraint, event
 
 from wce_crm import constants
 from wce_crm import types as t
@@ -420,6 +420,7 @@ zuordnung_personen_unternehmen: sql.Table = Table(
         UTCDateTime,
         nullable=True,
     ),
+    UniqueConstraint("un_id", "pers_id", name="uq_un_id_pers_id"),
 )