Create a "group key" for duplicate transactions

TYIA -
I'm working on a duplicate transaction analysis. I want to create a field where each duplicate "group" has a unique number. Any ideas?
Example of what I'm hoping for:
RecNo Vendor Date Amount **Group Key**
1             XYZ      4/20   $20        1
2             XYZ      4/20   $20        1
3             LMN      4/19   $35       2
4             LMN      4/19   $35       2