diff --git a/backend/src/main/resources/db/migration/V57__add_tbmp_unique_constraint.sql b/backend/src/main/resources/db/migration/V57__add_tbmp_unique_constraint.sql
new file mode 100644
index 00000000..b1945c73
--- /dev/null
+++ b/backend/src/main/resources/db/migration/V57__add_tbmp_unique_constraint.sql
@@ -0,0 +1,5 @@
+-- Prevent duplicate sidecar rows for the same (block, person) pair.
+-- @ElementCollection uses DELETE+INSERT per update so normal JPA writes can't
+-- create duplicates, but a raw-SQL import or concurrent bypass of JPA could.
+ALTER TABLE transcription_block_mentioned_persons
+    ADD CONSTRAINT uq_tbmp_block_person UNIQUE (block_id, person_id);