I have this in version 2.78, and it was also seen in two previous versions as well.
When opening an audio mp3 created with audacity, and the year is entered in the meta data already, mp3tag displays it as year//year. For example, if Audacity saves an mp3 file with the year 2016 entered in the meta data, mp3 tag will show 2016//2016 in the year box.
No other box to enter information has this same result, its just the year that does this.
It can be edited easily enough, but its something that should not happen.