Error creating a ModelSingleton record
[At create a Singleton record, this is created and updated with the same data.](https://foss.heptapod.net/tryton/tryton/-/blob/11d9848aa118416ece34bd133b6420074370eb17/trytond/trytond/model/modelsingleton.py#L67) In my Singleton **only** have a o2m with a field required and unique in target. For this, is impossible create the record because the **linked** `write` method, the `create` call, and `super().write` contains the same data: `'o2m_field': [['create', [{'field1': 'foo', 'user': 1}]]]` As one of the O2M fields is unique, do not allow save/create
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