Here is how to reproduce the problem on a MAC:

yorosiku (よろしく) onegai (お願い) itasimasu (いたします。)

These are the letters (and corresponding Japanese) we type to write the sentence.
We first type these three segments all together and press the Shift key to change the
characters of each segment, but if we press the Shift more than once (pressing Shift more than
once is necessary to get the right characters), it automatically copies and pastes or erases
random segments.