Transaction Scope inserting duplicate values. How to lock se

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.