Multiple Instances Of A Name?

I have a large dataset with a company name in Column A and a Town in Column B, e.g.

ABC Company London
ABC Company London
XYX Company Swindon
STJ Company Bristol
STJ Company Bristol
STJ Company Bristol
DEF Company London
DEF Company London
DEF Company London

and I need to identify every instance where the same town appears for each instance of each individual company and add in Column C incrememnt the town name, so the above should end up looking like this:

ABC Company London London
ABC Company London London-1
XYX Company Swindon Swindon
STJ Company Bristol Bristol
STJ Company Bristol Bristol-1
STJ Company Bristol Bristol-2
DEF Company London London
DEF Company London London-1
DEF Company London London-2

I'm thinking I need to use some sort of lookup, but because of the volume of the instances of each company I have no idea how many instance appear in the data set. How could I go about achieving this and incrementing the town names in Column C automatically for each instance?