isHolidays returns the last date instead of a list

[westn]

Problem:
On the swedish locale (SE), there's mothers day and pentecost occuring on the same date in 2020, see snippet taken from holidays.getHolidays('2020') below.

 {
    date: '2020-05-31 00:00:00',
    start: 2020-05-30T22:00:00.000Z,
    end: 2020-05-31T22:00:00.000Z,
    name: 'Pingst',
    type: 'public',
    rule: 'easter 49'
  },
  {
    date: '2020-05-31 00:00:00',
    start: 2020-05-30T22:00:00.000Z,
    end: 2020-05-31T22:00:00.000Z,
    name: 'Mors dag',
    type: 'observance',
    rule: 'sunday before 06-01'
  },

holidays.isHoliday(new Date('2020-05-31 00:00:00')) returns the snippet below.

{
  date: '2020-05-31 00:00:00',
  start: 2020-05-30T22:00:00.000Z,
  end: 2020-05-31T22:00:00.000Z,
  name: 'Mors dag',
  type: 'observance'
}

Expected behaviour: Return a list of holidays.

Research:

• Does this affect more locales?
• Is this an expected behaviour or broken?