What happens if accidentally a husband and wife each submit a specific charge to their respective FSA for reimbursement?
Search the website. There is generally a way to reverse the charge. I have seen these options exist on both Flexible spending accounts and Health Savings accounts.
If the expense was for last year, and you had other expenses that you did not submit because you reached the limit, you will probably be OK. Send them both information on the wrong submission on the new submission. If you left money on the table last year, they will want a check from you.
If the expense was for this year, you will not have a problem reversing the charge, because much of the year is left. Of course due to the new rules regarding roll-over of lat years money into this year it could be more complex.
You want to resolve it as soon a possible to minimize the complexity as deadlines for submission approach.
If you don't report the mistake the extra income from the incorrect submission is considered taxable.