The default arrayFormat value of none is problematic because arrays of length 1 are serialised to the same format as if the value was the string contained within the array.

For example:

qs.parse(qs.stringify({a: 'val'})); // {a: 'val'}
qs.parse(qs.stringify({a: ['val']})); // {a: 'val'} WRONG/UNEXPECTED
qs.parse(qs.stringify({a: ['val', 'val2']})); // {a: ['val', 'val2']}

Perhaps the default arrayFormat should be changed to bracket or index, which do not have this problem. The none option would make more sense IMO if it refused to stringify an array when stringifying, and ignored/crash for duplicate keys when parsing. I don't see how the comma option can be made to handle arrays consistently.