以下是一个示例代码，用于在Paypal上避免用户支付已经付款的商品：

import requests

def check_payment_status(payment_id):
    url = f"https://api.paypal.com/v1/payments/payment/{payment_id}"
    headers = {
        "Content-Type": "application/json",
        "Authorization": "Bearer YOUR_PAYPAL_ACCESS_TOKEN"
    }
    response = requests.get(url, headers=headers)
    payment_status = response.json()["state"]
    return payment_status

def process_payment(payment_id, amount):
    payment_status = check_payment_status(payment_id)
    if payment_status == "approved":
        # 商品已经支付
        return "Payment has already been completed."
    else:
        # 继续处理支付
        # ...
        return "Payment processed successfully."

# 示例使用
payment_id = "YOUR_PAYMENT_ID"
amount = 100.0
result = process_payment(payment_id, amount)
print(result)

在上面的示例中，check_payment_status函数用于检查给定支付ID的支付状态。该函数通过调用Paypal API来获取支付状态，需要替换YOUR_PAYPAL_ACCESS_TOKEN为您的Paypal访问令牌。

process_payment函数用于处理支付请求。它首先调用check_payment_status函数来检查支付状态。如果支付状态为“approved”（已批准），则返回“Payment has already been completed.”；否则，继续处理支付逻辑。

请注意，这只是一个示例代码，您需要根据您的具体需求和Paypal API进行适当的调整和扩展。