I need help flagging duplicate records in ome tables I have.

For example if I have Table1 which conatins Field1, Field2 and Field3
like below

Field1 Field2 Field3 Field4
Paul 18 Null Null
Paul 18 Null Null
John 19 Null Null

How would I;
1. put a 'Y' in Field3 to mark the two records which are duplicates.
2. put a 'Y' in Field4 to mark ONLY ONE of the duplicate records.

Regards,

Ciarán