Find a custom field values 'rank'?

I’d like to find the ‘rank’ of a custom field … but as I’m not even sure of the ‘short version’ of the question, I’m not having any luck ‘Googling’ for ideas.

Putting it as briefly / simply as possible:

I have a custom field – “hostcity” to which a value is assigned for each post. Sometimes this value is duplicated … e.g. “Dublin”. I (think I) can work out how via a php script to count that “Dublin” appears six times in total amongst all of the posts. Anyone know how I could go about writing a script that looks at an individual instance to say “This is the third time Dublin has been the host city”?

Not looking for a complete answer (though that would be brilliant, naturally!), just to be pointed in the right direction.

Is what I want to do impossible? Or will end up so long and complicated that I really should go away and get a life? Or is it easy once I know the right question?

Thoughts / suggestions / guidance would be very welcome.

(Can’t point to an individual page with an example, not that I think that would actually help, because at the moment I’m developing this part of my site offline.)