Fannie Mae has told lawmakers that it is working hard to improve internal detection systems after getting stung in a bogus mortgage fraud case. The government-sponsored enterprise was forced to pay the government $7.5 million late last year because First Beneficial Mortgage Co. repurchased bogus loans from Fannie with funds the lender received from Ginnie Mae. The case made headlines after lawmakers contended Fannie knew First Beneficial had passed off the bogus loans to Ginnie,…