Find by Pk should throw if there are more than one matched

Hi,
I've replaced 'FirstOrDefault()' by 'SingleOrDefault()'
I've run tests for MSSQL and all passed, shouldn't affect PG tests.

Because Find fetches item by primary key it should throw exception if there are more items matched and so developer is notified that there is some mess and could fix it. 
There could be several reason:
 * Id is not really Pk in DB 
 * (s)he missplaced PrimaryKey attribute
 * other property than intended was assigned as Pk by convention

The earlier one know that something is wrong, the better ;)