struct Account {
    let currencyCode: String
    let openDate: Date
}

let currencyCodes = ["RUR", "USD", "EUR"]

func sortedAccounts(_ accounts: [Account]) -> [Account] {

    var dates: [String: NSMutableOrderedSet] = [:]
    var duplicates: [String: [Date: Int]] = [ // it's not clear wether we can have identical records in array so...
        "RUR": [:],
        "USD": [:],
        "EUR": [:]
    ]

    for account in accounts {
        let set = dates[account.currencyCode] ?? NSMutableOrderedSet()
        if set.contains(account.openDate) {
            var counts = duplicates[account.currencyCode]
            let count = counts?[account.openDate] ?? 0
            counts?[account.openDate] = count + 1
        }
        set.add(account.openDate)
        dates[account.currencyCode] = set
    }

    var result: [Account] = []

    for code in currencyCodes {

        guard
            let set = dates[code],
            let array = Array(set) as? [Date] else {
            continue
        }

        for date in array {
            let duplicatesForCode = duplicates[code]
            let duplicatesCount = duplicatesForCode?[date] ?? 0

            (0..<duplicatesCount + 1).forEach { _ in
                result.append(Account(currencyCode: code, openDate: date))
            }
        }
    }

    return result
}