Once, unfortunately for a sweet fun loving hen named Betty, 4 "eggs" were laid in one day by her. They had backed up due to the first one in line being malformed and once that came out, the others followed quickly on its heels (1 more malformed- a rubbery collapsed twisted shell with no content - and one soft shelled and one hard shelled). In the ensuing months, she continued to have problems and eventually infections that at first responded to antibiotics and later did not and then sadly, I lost her shortly after a surgery intended to relieve her of the problem (hysterectomy). But anyway, long story short, it is most certainly possible for a hen to lay more than one egg in a day. And hopefully, not necessarily because of a problem like Betty's.