Power query formula needed

 I need to compare multiple rows for matches in the same column and add a suffix when the rows have the same information. 

In the figure below you can find my column "Nom", it can be repeated several times and I woudl like to add suffix "A", "B", "C", etc. for the same "Nom" number and leave it as it is if it doesn't repeat. 

I appreciate your help, I've been trying for several hours