Find missing numbers in list, ignore duplicates
 

I have database of addresses sorted by Surnames. Col C contains a Receipt
number for an annual sub. I sort the database weekly by Receipt number to
detect missing numbers. The numbers run from 1 to 246 and 521 to 566. Each
week numbers will have been added to both ranges. I have tried this formula
=SMALL(IF(ISNA(MATCH(ROW($2:$246),C$2:C$246,0)),RO W($2:$246)),ROWS($1:1))
However 21 couples have the same number which returns 247 thru 266 as
missing. I could try adding A to one partners number (83 83A) but that puts
them out of sequence, at the bottom of the list.
Not sure how to cope with the 500 series. At present it sorts as 1 thru 246
then 521 thru 566.
Any ideas appreciated. Thanks
--
Gilbert