Assining individual values to multiple identical strings

Hi all

Hoping someone can help here, I'm sure there's a really simple answer to this but I can't quite figure it out.

Basically I have a spreadsheet that contains a lot of replicated data. I need a way of assigining a number to each value that is the same; kind of like a count, but not the total number.

For example, I may have the word "monkey" appear 20 times. Can I make it so next to the first "monkey" it will show 1, the second 2, third 3 and so on...

Thanks in advance!