Add a character to duplicate emails using bash only

Input data:

id,location_id,name,title,name@zzz.com,department
1,1,Susan houston,Director of Services,shouston@zzz.com,
2,1,Christina Gonzalez,Director,cgonzalez@zzz.com,
3,2,Brenda brown,"Director, Second Career Services",bbrown@zzz.com,
4,3,Howard Lader,"Manager, Senior Counseling",hlader@zzz.com,
8,6,Bart charlow,Executive Director,bcharlow@zzz.com,
9,7,Bart Charlow,Executive Director,bcharlow@zzz.com,

I need to add a character to duplicate emails after the email part, i.e. bcharlow@zzz.com would become bcharlow7@zzz.com (the digit after the email part needs to be taken from the second column). How can I do that in Bash for all entries?