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<pre>
DataFrame customers
+-------------+--------+
| Column Name | Type |
+-------------+--------+
| customer_id | int |
| name | object |
| email | object |
+-------------+--------+
</pre>
<p>There are some duplicate rows in the DataFrame based on the <code>email</code> column.</p>
<p>Write a solution to remove these duplicate rows and keep only the <strong>first</strong> occurrence.</p>
<p>The result format is in the following example.</p>
<p>&nbsp;</p>
<pre>
<strong class="example">Example 1:</strong>
<strong>Input:</strong>
+-------------+---------+---------------------+
| customer_id | name | email |
+-------------+---------+---------------------+
| 1 | Ella | emily@example.com |
| 2 | David | michael@example.com |
| 3 | Zachary | sarah@example.com |
| 4 | Alice | john@example.com |
| 5 | Finn | john@example.com |
| 6 | Violet | alice@example.com |
+-------------+---------+---------------------+
<strong>Output: </strong>
+-------------+---------+---------------------+
| customer_id | name | email |
+-------------+---------+---------------------+
| 1 | Ella | emily@example.com |
| 2 | David | michael@example.com |
| 3 | Zachary | sarah@example.com |
| 4 | Alice | john@example.com |
| 6 | Violet | alice@example.com |
+-------------+---------+---------------------+
<strong>Explanation:</strong>
Alic (customer_id = 4) and Finn (customer_id = 5) both use john@example.com, so only the first occurrence of this email is retained.
</pre>