counting unique customers in a list by a specific date range

[DIFFICULT] I have a list of customers that have made orders on several different dates. I want to count the number of unique customers for a specific date range.

the equation I use below doesn't work and I'm not sure why.

=SUM(IF(AND(FREQUENCY(MATCH(B2:B13,B2:B13,0),MATCH(B2:B13,B2:B13,0))>0,E2:E13>=K3,E2:E13<=K4),1,0))

B2:B13 refers to the customers
E2:E13 are the dates of the orders
K3 and K4 are the date range

See the attached spreadsheet, thanks