The answer is yes although your use of the word "randomly" is not appropriate. This is a big enough issue that it is part of a specific section of UA's COC (Rule 5(2)(H)). Other carriers have similar provisions.
While the COC permits UA to simply cancel without notice, as a general matter, you will receive a call, email or your itinerary will show up with a big red notice to contact UA and you will be asked to cancel one ticket. But, you cannot count on that and if you book through a third-party such as a TA, OTA, third-party site, corporate TA or whatever, the notice goes to the third-party not to you.
This is all software-based and it is easy stuff. These are "impossible" bookings and there are varying degrees of "impossible". Some simply require verification (and people are grateful because they are likely errors) and other are ticketing fraud.
The refundable ticket issue is a non-starter. This is not why people purchase these tickets and it is not their intended purpose. You can search through FT and find rants from even GS who have had their status revoked and their MP accounts zeroed out and closed for purchasing and then cancelling at the last second (gate) on repeated occasions international F tickets (which are fully refundable).
The issue is simple, at the time you purchase a ticket, you must intend to fly it. If you hold two tickets and it is impossible to fly both of them as booked, you cannot possibly intend to fly them both.