Prevent UserIdentity metadata table from growing

CHANGELOG.md

@@ -16,6 +16,7 @@
 * Having links in a property of Mixed type would lead to ill-formed JSON output when serializing the database. ([#5125](https://github.com/realm/realm-core/issues/5125), since v11.0.0)
 * FLX sync QUERY messages are now ordered with UPLOAD messages ([#5135](https://github.com/realm/realm-core/pull/5135))
 * Fixed race condition when waiting for state change notifications on FLX subscription sets that may have caused a hang ([#5146](https://github.com/realm/realm-core/pull/5146))
+* UserIdentity metadata table grows indefinitely. ([#5152](https://github.com/realm/realm-core/issues/5152), since v10.0.0)
 
 ### Breaking changes
 * FLX SubscriptionSet type split into SubscriptionSet and MutableSubscriptionSet to add type safety ([#5092](https://github.com/realm/realm-core/pull/5092))

src/realm/object-store/sync/impl/sync_metadata.cpp

@@ -578,14 +578,23 @@ void SyncUserMetadata::set_identities(std::vector<SyncUserIdentity> identities)
     m_realm->begin_transaction();
 
     auto link_list = m_obj.get_linklist(m_schema.identities_col);
-
+    auto identities_table = link_list.get_target_table();
+    auto col_user_id = identities_table->get_column_key(c_sync_user_id);
+    auto col_provider_type = identities_table->get_column_key(c_sync_provider_type);
     link_list.clear();
 
-    for (size_t i = 0; i < identities.size(); i++) {
-        auto obj = link_list.get_target_table()->create_object();
-        obj.set<String>(c_sync_user_id, identities[i].id);
-        obj.set<String>(c_sync_provider_type, identities[i].provider_type);
-        link_list.add(obj.get_key());
+    for (auto& ident : identities) {
+        ObjKey obj_key = identities_table->where()
+                             .equal(col_user_id, StringData(ident.id))
+                             .equal(col_provider_type, StringData(ident.provider_type))
+                             .find();
+        if (!obj_key) {
+            auto obj = link_list.get_target_table()->create_object();
+            obj.set<String>(c_sync_user_id, ident.id);
+            obj.set<String>(c_sync_provider_type, ident.provider_type);
+            obj_key = obj.get_key();
+        }
+        link_list.add(obj_key);
     }
 
     m_realm->commit_transaction();