In order to work with the unique constraints you will need to remember the following 3 steps.
1. Add an index for a field you wish to be unique:
c#:
configuration.ObjectClass(typeof(Item)).ObjectField("field").Indexed(true);
VB:
configuration.ObjectClass(GetType(Item)).ObjectField("field").Indexed(true)
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2. Configure a unique constraint:
3. Handle unique constraint violation: