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> I'm not an expert either in statistics or in polling

Fortunately for us, the author is :)

The author has an outline available for a course on survey sampling for those interested in references and topics to learn more. http://www.stat.columbia.edu/~gelman/surveys.course/surveys_...

It certainly could and depends on the questions being asked. If you are sending out a questionnaire via email and some of the questions are along the lines of "how many times a day do you check your email?" or "how much time do you spend at your computer?" you could certainly get some skewed results.

Of course, it would not be a random sample if 75% get one questionnaire and 25% get two questionnaires and encouragement.

Probably close enough and potentially closer just sending out one request.