In C#, SQLite randomly throws "Constraint failed\r\nColumn ... is not unique" -

i'm using entityobjects in c# , backend sqlite. system information:

windows xp (up-to-date), vs2010 premium, system.data.sqlite 1.0.88.0 (3.7.17) 

the table schema is:

create table [transaction] (   [primary_key_col] integer primary key autoincrement,    [transaction_code] int not null,    [account_code] int not null,    [category_code] int not null,   [transaction_date] date not null,   [description] varchar(50)); 

public partial class transaction : entityobject represents table.

to add new transaction, call following code:

transaction transaction = new transaction() {     description = _desc,     transaction_code = generatehash(_desc),     account_code = _acccode,     category_code = _catcode,     transaction_date = datetime.now };  dbentities.transactions.addobject(transaction);  dbentities.savechanges(system.data.objects.saveoptions.acceptallchangesaftersave); 

works fine after adding few rows, following exception:

constraint failed\r\ncolumn account_code not unique. 

if exit application , restart it, error goes away randomly comes again.

i know account_code not unique, never asked unique!!!

there 3 specific ways application can create transaction , exception thrown rather randomly of these methods. sqlite? (i'm new sqlite). idea why happening? i've pulled out hair ...

as per comment seems have done trick:

are using "new" dbentities context every time insert or re-using same 1 multiple inserts? shouldn't matter either way try former if using latter.