Обсуждение: group number

Thank you very much for your attention.

I'm facing a mental block to achieve this in a performant manner:

Input:

a

a

b

c

c

c

d

Desired output:

a 1

a 1

b 2

c 3

c 3

c 3

d 4

Any suggestions? Have a good day (night, afternoon or evening) and greetings from México City.