two rows are inserted by SELECT execq('INSERT INTO a SELECT x + 2FROM a', 1)

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Page: https://www.postgresql.org/docs/9.6/static/spi-examples.html
Description:

I did some testing following this page:
https://www.postgresql.org/docs/9.6/static/spi-examples.html.

I got a different result. 

According to the webpage, only one row is inserted by following SQL: 

SELECT execq('INSERT INTO a SELECT x + 2 FROM a', 1);

But I ran this SQL and two rows were inserted.