try {
                if ( !dupsAllowed && dropDups ) {
                    LastError::Disabled led( lastError.get() );
                    btBuilder.addKey(d.first, d.second);
                }
                else {
                    btBuilder.addKey(d.first, d.second);                    
                }
            }
            catch( AssertionException& e ) {
                if ( dupsAllowed ) {
                    // unknown exception??
                    throw;
                }

                if( e.interrupted() ) {
                    killCurrentOp.checkForInterrupt();
                }

                if ( ! dropDups )
                    throw;

                /* we could queue these on disk, but normally there are very few dups, so instead we
                    keep in ram and have a limit.
                */
                dupsToDrop.insert(d.second);
                uassert( 10092 , "too may dups on index build with dropDups=true", dupsToDrop.size() < 1000000 );
            }