F# Runs in immediate window, but doesn't build

Question

I've got the tutorial template in Visual Studio open. There aren't any errors in the error window, but when I build I get this exception:

FSC: error FS2014: A problem occurred writing the binary ...\obj\Debug\Junk.FSharp.TutorialTemplate.exe': Error in pass2 for type Tutorial, error: Error in pass2 for type DiscriminatedUnions, error: Error in pass2 for type Shape, error: duplicate entry '.ctor' in method table

The DiscriminatedUnion bit is:

/// Discriminated Unions (DU for short) are values which could be a number of named forms or cases.
/// Data stored in DUs can be one of several distinct values.
///
/// To learn more, see: https://docs.microsoft.com/en-us/dotnet/articles/fsharp/language-reference/discriminated-unions
module DiscriminatedUnions = 

    /// The following represents the suit of a playing card.
    type Suit = 
        | Hearts 
        | Clubs 
        | Diamonds 
        | Spades

    /// A Disciminated Union can also be used to represent the rank of a playing card.
    type Rank = 
        /// Represents the rank of cards 2 .. 10
        | Value of int
        | Ace
        | King
        | Queen
        | Jack

        /// Discriminated Unions can also implement object-oriented members.
        static member GetAllRanks() = 
            [ yield Ace
              for i in 2 .. 10 do yield Value i
              yield Jack
              yield Queen
              yield King ]

    /// This is a record type that combines a Suit and a Rank.
    /// It's common to use both Records and Disciminated Unions when representing data.
    type Card = { Suit: Suit; Rank: Rank }

    /// This computes a list representing all the cards in the deck.
    let fullDeck = 
        [ for suit in [ Hearts; Diamonds; Clubs; Spades] do
              for rank in Rank.GetAllRanks() do 
                  yield { Suit=suit; Rank=rank } ]

    /// This example converts a 'Card' object to a string.
    let showPlayingCard (c: Card) = 
        let rankString = 
            match c.Rank with 
            | Ace -> "Ace"
            | King -> "King"
            | Queen -> "Queen"
            | Jack -> "Jack"
            | Value n -> string n
        let suitString = 
            match c.Suit with 
            | Clubs -> "clubs"
            | Diamonds -> "diamonds"
            | Spades -> "spades"
            | Hearts -> "hearts"
        rankString  + " of " + suitString

    /// This example prints all the cards in a playing deck.
    let printAllCards() = 
        for card in fullDeck do 
            printfn "%s" (showPlayingCard card)