How do I...Transaction Scope inserting duplicate values. How to lock select query

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ASP.NET

SQL

C#3.5

T-SQL

MVC4

EF5.0

i am using transaction scope in my project. Isolation level is IsolationLevel.ReadCommitted

i Also tried IsolationLevel.RepeatableRead

using (DBOperations dboperation = new DBOperations())
    {
      GetAccountVoucherNuber();
    }

i am calling this method from Transaction Scope.

private void GetAccountVoucherNuber() {

    var _dB0010013 = _dB0010013Repository.GetAll().Where(e = > e.Category == "D" && e.CategoryType == "TSNO" && e.Branch == Branch && EntityFunctions.TruncateTime(e.OnDate) == EntityFunctions.TruncateTime(TranInit.EntryDate) && e.CodeOne == BatchCode).FirstOrDefault();
    if (_dB0010013 == null) {
        _dB0010013 = new DB0010013();
        _dB0010013.Branch = Branch;
        _dB0010013.Category = "D";
        _dB0010013.CategoryType = "TSNO";
        _dB0010013.CodeOne = BatchCode;
        _dB0010013.CodeTwo = "";
        _dB0010013.CodeThree = "";
        _dB0010013.OnDate = TranInit.EntryDate;
        _dB0010013.Note = "";
        _dB0010013.LastNo = 1;

        var _operationStatus = _dB0010013Repository.AddAndSave(_dB0010013, false);
    } else {
        _dB0010013.LastNo += 1;
        var _operationStatus = _dB0010013Repository.UpdateAndSave(_dB0010013, false);
    }

}

when two or more user submits the page at same time.
i am getting same no for two transaction.


eg. suppose user1 got lastNo =85 +=1 =86

at the same time user2 also get lastNo =85 +=1 =86

so the same no is applied for two different transactions.


How Can i Lock Select Statement after getting values. or what is way to handle it.
i googled a lot but didnt find any solution.