Check where you see that. If it is in the columns of MP3tag then have a look at the value definition of that column. It may be that you get to see multi-value fields in one column. The double-backslash is the indicator used by MP3tag to show multi-value fields.
You can also check the presence of multi-value fields in the extended tags dialogue (Alt-T): if you see several fields with the same name, then you have a multi-value field.
I would say there is nothing wrong.