The double backslash \\ is Mp3tag's way of visually denoting multiple values for the same field. For example, Peter Gabriel\\Kate Bush in the ARTIST field indicates that there are two separate ARTIST values: Peter Gabriel and Kate Bush.
You can inspect these values via View → Extended Tags... or by pressing Alt+T, where the individual field–value pairs are listed.
If you want to combine multiple values into a single value separated by a different character sequence, you can use the Merge duplicate fields action and specify a separator such as ; .
Alternatively, you can use Convert → Tag - Tag or the Format value action with a format string such as $meta_sep(artist,; ). The $meta_sep() function returns all values of the specified field separated by the given separator.
Note: Not all players support multiple separate field values or multiple values combined into a single field. Compatibility varies between applications, so it's best to test what works for your setup.