[Metadata composite keys] Validate that primary key columns are unique

[frances-h]

Environment Details

Please indicate the following details about the environment in which you found the bug:

• SDV version:
• Python version:
• Operating System:

Error Description

Passing the same column multiple times to set_primary_key results in a confusing error during fit: AttributeError: 'DataFrame' object has no attribute 'dtype'.

When validating the primary key, we should error if the same column name appears multiple times.

Steps to reproduce

import pandas as pd
import numpy as np
from sdv.metadata import Metadata

account_metadata = Metadata.load_from_dict({
    'tables': {
        'accounts': {
            'columns': {
                'user_id': {'sdtype': 'id', 'regex_format': 'ID_[0-9]{1,2}'},
                'account_type': {'sdtype': 'categorical'},
                'col1': {'sdtype': 'categorical'},
                'col2': {'sdtype': 'categorical'},
            }
        }
    }
})
account_metadata.set_primary_key(['user_id', 'user_id'], table_name='accounts')
account_metadata.validate()