Countif function goes crazy

Hi all,
I have a set of about 30K records (and a strong enough machine to handle processing data). Some of the records are duplicates, so in order to clean up and understand the data better, here is what I did in order to get a frequency table of the duplicates:

1. Copied the data to a new sheet, performed "remove duplicates".
2. In column B, next to each value in this short list, I ran a countif function, with =countif(range in the original sheet,value in the short list).
I remembered to ctrl+shift+enter and dragged the formula to all values.

However, the sum of column B is GREATER than the number of records in the original file......... I don't understand how that could be!
Another strange thing is that everytime I run it, one value (couldn't find any pattern as to which value), gets an absurdly high count, such as 3170.
(However, even with excluding this high number, the total count still doesn't match).

Anyone had any similar experiences?
Can you suggest what happened?

Thanks a lot

<< going to resume banging my head against a wall >>

P.S. I tried converting all the values to text, but got the same results.