@Mark Barrett,
I'm not sure what BT is - is it British Telecom?
There may be a setting, where you are getting mail in both Yahoo and through your provider, and then the provider is shunting their copy through Yahoo as well. That could explain both the dupes and the slowness.
I'd try calling the provider first, see if they can hammer some of it out. Then go to Yahoo help if that does not work (I know they're not that helpful; you'll need to hunt around for items on duplications, email forwarding, etc.).