You know the character card decks, that players choose after picking a hero of that class? Like the "Berserker" deck for the melee characters? Are there expansions to those (either additional decks or additional cards for the existing decks)?
No there are no additional decks for any specific class "berserker" "knight" etc.
Both the berserker and knight are "warriors" and with every expansion, new "warrior" "scout" "healer" and "mage" classes are released, each with their own deck of cards.
If not are any planned/possible?
Not that i am aware of, the decks are balanced to last a whole campaign so adding more into them might get pretty unbalanced?
Are there extra copies of them for sale so if two players wanted to play that same style character they could?
I would doubt you can buy extra decks, however if you want to run 2 berserkers what is to stop you doing so with pen and paper? All you need to know are which skills each one takes.
Some classes that have their own familiars/ tokens (like the geomancer / necromancer could be trickier but again just proxying would be possible.
I for one would like to see a campaign with 4 geomancers, although im sure half of the quests would be broken beyond repair with chokepoints!