Feature: avoid duplicates for the same period in acc period user history

.changes/unreleased/Feature-20230627-151615.yaml

@@ -0,0 +1,28 @@
+kind: Feature
+body: "Force accompanying period user history to be unique for the same period and
+  stardate/enddate [:warning: may encounter migration issue]"
+time: 2023-06-27T15:16:15.775571488+02:00
+custom:
+  Issue: ""
+  Long: "If some issue is encountered during migration, use this SQL to find the line which are in conflict, examine the problem and delete some of the concerning line
+
+    ```sql
+
+    -- to see the line which are in conflict with another one
+
+    SELECT o.*
+
+    FROM chill_person_accompanying_period_user_history o
+
+    JOIN chill_person_accompanying_period_user_history c ON o.id < c.id AND o.accompanyingperiod_id = c.accompanyingperiod_id
+
+    WHERE tsrange(o.startdate, o.enddate, '[)') && tsrange(c.startdate, c.enddate, '[)')
+
+    ORDER BY accompanyingperiod_id;
+
+    -- to examine line in conflict for a given accompanyingperiod_id (given by the previous query)
+
+    SELECT * FROM chill_person_accompanying_period_user_history WHERE accompanyingperiod_id = IIIIDDDD order by startdate, enddate;
+
+    ```
+    "