Dessie Robinson, a 55-year-old welfare recipient who suffers from a heart ailment, diabetes and ulcers, had just finished showering in her room at a downtown homeless shelter early one morning in July when she heard a loud knock at the door. Throwing on some clothes, she opened the door and was confronted by half a dozen armed federal agents who proceeded to arrest her on a misdemeanor charge of cheating the government out of $746.10, the monthly payment she receives under the federal program to aid the blind, disabled or elderly poor. Robinson was among 21 people arrested at gunpoint that week during a sweep targeting individuals who said they hadn't received their monthly Supplemental Security Income checks, but who allegedly cashed the original and the duplicate checks, according to prosecutors. Most of those taken into custody were physically or mentally disabled and frightened out of their wits as they were handcuffed and hauled off to court, according to lawyers from the federal public defender's office.