appendInFolder method is wrong, it results duplicate add menuitems

[andy380743909]

https://github.com/eleev/expandable-collection-view-kit/blob/e3cc00bacff3a3624a290e4e2afd20710ed66aaf/Sources/ExpandableCollectionViewKit/View%2BController/ExpandableCollectionViewManager.swift#L113

Xcode console warning

[UIDiffableDataSource] Diffable data source detected an attempt to insert or append 1 item identifier that already exists in the snapshot. The existing item identifier will be moved into place instead, but this operation will be more expensive. For best performance, inserted item identifiers should always be unique. Set a symbolic breakpoint on BUG_IN_CLIENT_OF_DIFFABLE_DATA_SOURCE__IDENTIFIER_ALREADY_EXISTS to catch this in the debugger. Item identifier that already exists: <invalid NS/CF object>