I am making a client register for a homeless shelter. I get a total of individual clients at the end of each month. I do not want to count repeat clients twice. The clients names are in column C6:C36. For this I use the formula:

=SUMPRODUCT((C6:C36<>"")/COUNTIF(C6:C36,C6:C36&""))


I now need to develop a formula for the Quarterly report (separate worksheet) that gives only the individuals for the 3 month period (each month is on a separate worksheet) So if someone stayed in January and again in March, they should only be counted once. Is there a formula that can give the total number of individuals from 3 separate columns on different worksheets without counting repeat clients?