Welcome back! Unique is addressed on page 15 of the Compendium. The new rulebook for Weird West Edition included in the Kickstarter active right now will address this and replace our online version.
Dudes and deeds in Doomtown are mostly unique; any exceptions have the keyword “NonUnique” on them. This means you can put more than one copy of any card in your deck, but you can’t bring a Unique card into play if you own another card with the same name that is either already in play or in your Boot Hill. If you have a Unique card in play, that doesn’t stop someone else from having that same Unique card in play. However, you check for uniqueness at the point of entering play. If a Unique card is in play, and another copy goes to Boot Hill, this has no effect on the in-play card. Other card types are not considered Unique unless they have the Unique keyword.