This is an old thread but a new problem for me (ipad air, IOS 12.0). My screen keyboard started repeating letters rather randomly with one press, as in raandoomlly... I tried the recommended fixes: reboot ipad, turn off keyboard settings, delete/change keyboards etc. Nothing worked. I saw one rather strange recommendation: disconnect the charging cable. It is a Lightning cable. Disconnecting it cured the problem. I did assorted typing tests with the cable plugged in and unplugged. The cable plugged in is the culprit. I have had to replace the charging cable recently, with a third party one. The problem may be a result of this third party cable, maybe not. However this solution worked for me.