Unique formula that also counts non-unique once

I need to find unique IDs and the first or one instance of non-unique and dates using the below sample set of data.

I was using a UDF that used find to count the first instance of a non-unique id+date but for a large amount of data this is slow and was also unreliable. If an ID has a unique ID and date it gets a "Y" if an ID is non-unique with a unique date it gets a "Y", if an ID is non-unique with a non-unique date it needs to be counted once with "Y" and the other of its matches as "N". Also needs to account for "[NULL]" dates.

The final column is an sample of results I would like to see. I can include my UDF but it did not work well and I would like to see if someone has any insight beyond that.

ID Date Unique Formula
10001 1/1/2012 Y
10002 2/30/2012 Y
10030 4/30/2011 Y
40000 6/10/2012 Y
10001 1/1/2012 N
10002 1/3/2012 Y
40000 6/10/2012 N
12345 [NULL] Y
10001 1/1/2012 N

Ron. this is beautiful!