added check for duplicate cards

[rglerum]

Fixes #287

Tested on the new spoilers; master has multiple copies of the new cards, this resolves that. Also tested on KHM w/ double-sided cards.

Master:

    <card>
      <name>Alrund, God of the Cosmos // Hakka, Whispering Raven</name>
      <text/>
      <prop>
        <type>Legendary Creature — God // Legendary Creature — Bird</type>
        <maintype>Creature</maintype>
        <cmc>5</cmc>
      </prop>
      <set rarity="Mythic Rare" picURL="https://cards.scryfall.io/normal/front/b/7/b751cf69-0a02-4cd2-abd4-cdb65ca620a8.jpg?1631054227">KHM</set>
      <tablerow>2</tablerow>
    </card>
    <card>
      <name>Alrund, God of the Cosmos // Hakka, Whispering Raven</name>
      <text/>
      <prop>
        <type>Legendary Creature — God // Legendary Creature — Bird</type>
        <maintype>Creature</maintype>
        <cmc>5</cmc>
      </prop>
      <set rarity="Mythic Rare" picURL="https://cards.scryfall.io/normal/front/5/d/5d131784-c1a3-463e-a37b-b720af67ab62.jpg?1665157358">KHM</set>
      <tablerow>2</tablerow>
    </card>

Branch:

    <card>
      <name>Alrund, God of the Cosmos // Hakka, Whispering Raven</name>
      <text/>
      <prop>
        <type>Legendary Creature — God // Legendary Creature — Bird</type>
        <maintype>Creature</maintype>
        <cmc>5</cmc>
      </prop>
      <set rarity="Mythic Rare" picURL="https://cards.scryfall.io/normal/front/b/7/b751cf69-0a02-4cd2-abd4-cdb65ca620a8.jpg?1631054227">KHM</set>
      <tablerow>2</tablerow>
    </card>

[tooomm]

Hi @rglerum, did you have a chance to look into the review and comment from ebbit?

Do you plan to adjust your code for that? :)