Formula to count how many numbers repeat in a column.

My spreadsheet records admissions/discharges on a hospital floor. Each admission and its corresponding discharge are recorded in 1 row. The unique hospital card number of each patient is admitted in Column D (number can be anywhere from 4 to 8 digits).

What formula will yield the number of hospital card numbers that are repeated in Column D (the number of patients who have been admitted more than once)?

If Column D contains:
4444
4444
7777777
55555
4444
666666
88888888
7777777
then the formula should yield "2" (because the 2 numbers "4444" and "7777777" are repeated).

I think this is complicated enough for now. Eventually, though, I want to have cells that show how many patients have been admitted 2 times; how many have been admitted 3 times; etc. Hopefully I will be able to figure out those formulae on my own, referring to the cell I am asking about here.

Thanks in advance for any suggestions/advice,

- Simon