diff --git a/backend/src/database/migrations/V1775000000__fix-member-segment-affiliations.sql b/backend/src/database/migrations/V1775000000__fix-member-segment-affiliations.sql
new file mode 100644
index 0000000000..009a98dc69
--- /dev/null
+++ b/backend/src/database/migrations/V1775000000__fix-member-segment-affiliations.sql
@@ -0,0 +1,24 @@
+-- 1. Deduplicate existing memberSegmentAffiliations
+DELETE FROM "memberSegmentAffiliations" a USING (
+    SELECT MIN(id) as keep_id, "memberId", "segmentId", "organizationId", "dateStart", "dateEnd"
+    FROM "memberSegmentAffiliations"
+    GROUP BY "memberId", "segmentId", "organizationId", "dateStart", "dateEnd"
+    HAVING COUNT(*) > 1
+) b
+WHERE a."memberId" = b."memberId"
+AND a."segmentId" = b."segmentId"
+AND a."organizationId" IS NOT DISTINCT FROM b."organizationId"
+AND a."dateStart" IS NOT DISTINCT FROM b."dateStart"
+AND a."dateEnd" IS NOT DISTINCT FROM b."dateEnd"
+AND a.id <> b.keep_id;
+
+-- 2. Add an index to prevent exact duplicates in the future
+-- Using COALESCE ensures that NULL values are logically treated as equal 
+-- across all supported PostgreSQL versions for the sake of uniqueness.
+CREATE UNIQUE INDEX "uq_member_segment_affiliations" ON "memberSegmentAffiliations" (
+    "memberId",
+    "segmentId",
+    COALESCE("organizationId", '00000000-0000-0000-0000-000000000000'::uuid),
+    COALESCE("dateStart", '1970-01-01T00:00:00Z'::timestamptz),
+    COALESCE("dateEnd", '1970-01-01T00:00:00Z'::timestamptz)
+);
diff --git a/backend/src/database/repositories/memberSegmentAffiliationRepository.ts b/backend/src/database/repositories/memberSegmentAffiliationRepository.ts
index b55bf6d5ec..9f516c4a68 100644
--- a/backend/src/database/repositories/memberSegmentAffiliationRepository.ts
+++ b/backend/src/database/repositories/memberSegmentAffiliationRepository.ts
@@ -48,6 +48,14 @@ class MemberSegmentAffiliationRepository extends RepositoryBase<
       `INSERT INTO "memberSegmentAffiliations" ("id", "memberId", "segmentId", "organizationId", "dateStart", "dateEnd")
           VALUES
               (:id, :memberId, :segmentId, :organizationId, :dateStart, :dateEnd)
+          ON CONFLICT (
+              "memberId",
+              "segmentId",
+              COALESCE("organizationId", '00000000-0000-0000-0000-000000000000'::uuid),
+              COALESCE("dateStart", '1970-01-01T00:00:00Z'::timestamptz),
+              COALESCE("dateEnd", '1970-01-01T00:00:00Z'::timestamptz)
+          )
+          DO UPDATE SET "organizationId" = EXCLUDED."organizationId"
           RETURNING "id"
         `,
       {