add validation for schemas with duplicate Column names

[nirmit-damania-dyno]

This PR adds a validation rule to BaseModel which raises a ValueError on class init if columns exist with duplicate names. For example...

class Testschema(BaseModel, CustomEntityMixin):
    __schema_properties__ = ...

    test = Column(name="Test", type=BenchlingFieldType.INTEGER, required=False)
    test_two = Column(name="Test", type=BenchlingFieldType.INTEGER, required=False)

raises: ValueError: Schema cannot have fields with duplicate Column names. when running from ..schemas.entity_schemas import *