Args:
    invoices (list of dict): A list of dictionaries containing invoice data.
                             Each dictionary should have keys: 'invoice_id', 'vendor', 'amount', 'date'.

Returns:
    list: A list of flagged invoices with reasons for suspicion.
"""
flagged_invoices = []

# Helper function to check for duplicate invoice IDs
def has_duplicate_ids(invoices):
    ids = [invoice['invoice_id'] for invoice in invoices]
    return len(ids) != len(set(ids))

# Check for duplicate invoice IDs
if has_duplicate_ids(invoices):
    flagged_invoices.append("Duplicate invoice IDs found.")

# Check each invoice for anomalies
for invoice in invoices:
    reasons = []
    
    # Check for unusually high or low amounts
    if invoice['amount'] <= 0:
        reasons.append("Amount is zero or negative.")
    elif invoice['amount'] > 1_000_000:  # Example threshold for large amounts
        reasons.append("Unusually high amount.")

    # Check for invalid dates (e.g., future dates)
    from datetime import datetime
    invoice_date = datetime.strptime(invoice['date'], '%Y-%m-%d')
    if invoice_date > datetime.now():
        reasons.append("Future date detected.")

    # Check for repeated vendors with large amounts in a short period
    vendor_invoices = [inv for inv in invoices if inv['vendor'] == invoice['vendor']]
    if len(vendor_invoices) > 5:
        reasons.append("High frequency of invoices from the same vendor.")

    if reasons:
        flagged_invoices.append({
            'invoice_id': invoice['invoice_id'],
            'vendor': invoice['vendor'],
            'amount': invoice['amount'],
            'date': invoice['date'],
            'reasons': reasons
        })

return flagged_invoices