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Duplicates

property Duplicates: TDuplicates read write;

Example​

procedure ScriptEvent(var Value: variant);
var
  Names: TStringList;
begin
  Names := TStringList.Create;
  try
    Names.Duplicates := dupIgnore;
    Names.Sorted := True;
    Names.Add('Alpha');
    Names.Add('Alpha');
    Value := Names.Count;  // 1
  finally
    Names.Free;
  end;
end;

Usage​

Duplicates selects accept, ignore or exception behaviour for equal values added while maintained sorted mode is active.

Additional Technical Info​

Duplicates controls what Add/AddObject does when Find reports an equal String while Sorted is true:

  • dupIgnore returns the existing matching index without inserting, changing the attached object or firing change events;
  • dupAccept inserts another String/object pair at the comparator position; and
  • dupError raises EStringListError before mutation or change events.

The default is dupIgnore, but the property has no effect while Sorted=False: additions append and all duplicates are accepted. Calling Sort alone does not change that flag, so setting a duplicate policy plus calling Sort is insufficient for future enforcement.

The setter stores the enumeration directly. It does not scan, remove, merge or reorder existing values and fires no events. A list can therefore contain duplicates while the property says ignore or error—because they were added while unsorted, copied from another list, or became equal after CaseSensitive changed.

Equality uses the current case and hidden locale settings, not exact Unicode code-unit identity by default. Under case-insensitive comparison, Alpha and alpha can be duplicates. Locale changes can also alter equivalence/order between process runs.

When dupIgnore finds an existing String, the new AObject passed to AddObject is not stored or freed. Because parameterless script-created lists have unexposed OwnsObjects=False, the caller remains responsible for that object in every duplicate outcome. dupError likewise leaves it with the caller.

The source-reviewed example configures the policy before enabling sorted mode and confirms the ignored insertion. It was not executed by the documentation workflow.

External references

Created 2026-07-15