If you look at the label below the poster, you’ll notice that one has the year the movie was released (2012) while the other one says 1 Movie. The 1 Movie one is a collection.
Unless you have manually created this, it’s probably automatically created. With the Plex Movie agent, you can configure if you want Plex to automatically create collections and how many items there should be (minimum threshold; e.g. if you want collections but only if you own at least 2 items of that collection).
see: ⋮ > Manage Library > Edit... → Advanced > Minimum automatic collection size
If you want the collection but avoid this kind of “duplicate” situation, you can also configure your library to hide items which are part of a collection (this way you’ll e.g. only see one “Lord of the Rings” item instead of the 3 feature films)
see: ⋮ > Manage Library > Edit... → Advanced > Collections
