I had built a query in Knex.js to insert records to the DB

upsert(input) {
    if (isEmpty(input)) return Promise.resolve(0);

    return this.tx(tableName)
      .insert(input.map(prepareInput))
      .onConflict([columns.candidateId, columns.startAt])
      .merge()
      .returning()
      .then(({ rowCount }) => rowCount);
  }

But I'm getting this error

ON CONFLICT DO UPDATE command cannot affect row a second time

I need to insert data and merge it with the existing one and not sure how should I edit this query to avoid the error