diff --git a/database_scheme.sql b/database_scheme.sql
index a00a149..acafd36 100644
--- a/database_scheme.sql
+++ b/database_scheme.sql
@@ -49,5 +49,6 @@ CREATE TABLE "activities_and_emotions" (
   "type_id" integer REFERENCES types_of_activities_and_emotions NOT NULL,
   "day_id" integer REFERENCES days ON DELETE CASCADE NOT NULL,
   "proportion" integer NOT NULL,
+  UNIQUE (type_id, day_id), -- A day can't contain the same activity/emotion twice
   CONSTRAINT type_owned_by_correct_user_check CHECK (does_activity_or_emotion_belong_to_user_of_the_day(type_id,day_id))
 );
diff --git a/src/webhandlers.go b/src/webhandlers.go
index 7bb6200..4159d75 100644
--- a/src/webhandlers.go
+++ b/src/webhandlers.go
@@ -215,8 +215,13 @@ func HandleAPIDays(w http.ResponseWriter, r *http.Request) {
 				return
 			}
 
-			// TODO: also implement and document the handling of the case when the request contained activity/emotion
-			// multiple times
+			if pgErr.Constraint == "activities_and_emotions_type_id_day_id_key" {
+				writeJSON(w,
+					map[string]interface{}{"ok": false, "error": "Activity/emotion type id can't be mentioned more " +
+						"than once", "error_type": "duplicated_type"},
+					http.StatusBadRequest)
+				return
+			}
 		}
 		panicIfError(err)
 	}