Koha adds a comma to the relator terms, and since the standard is to include a comma in the record, this displays as double commas. For instance this: 100 1#$aAllepuz, Anuska, $d 1979- $e author, $e illustrator. shows as Allepuz, Anuska, 1979- [author,, illustrator.]
This is also a problem in the 700 fields--it's problematic for users because it implies that there's some non-displaying information between the double commas--the user doesn't know that they're seeing everything.
I agree with the previous comments. According to examples from Library of Congress and OCLC bibformats (that our cataloger was happy to provide), the records are correct to use the punctuation in these spaces. So, it would be nice for Koha to display as written.