Why is my script misbehaving when it spawns players?

Basically, you are getting clothing for the zombie every time it loops through a piece of the character because you have it in the for loop.

Removing it from the for loop should make it only clone one pair of clothing.

local Players = game:GetService("Players")

local function onCharacterAdded(character)
    Player = game.Players:GetPlayerFromCharacter(character)
    wait(0.5)
    if Player.PlayerParameters.HumanOrZombie.Value == 0 then 
        -- no spawn required

    elseif Player.PlayerParameters.HumanOrZombie.Value == 1 then
        -- human!
        wait(0.5)
        survivorspawns = workspace:WaitForChild("SurvivorSpawns"):GetChildren()
        randomspawn = survivorspawns[math.random(1, #survivorspawns)]
        character:MoveTo(randomspawn.Position)

    elseif Player.PlayerParameters.HumanOrZombie.Value == 2 then
        -- zombie!
        for i,v in pairs(character:GetChildren()) do
            if v:IsA("Accessory") then
                v:Destroy()
            elseif v:IsA("BasePart") then
                v.BrickColor = BrickColor.new("Dark green")
            elseif v:IsA("Shirt") then
                v:Destroy()
            elseif v:IsA("Pants") then
                v:Destroy()
            elseif v:IsA("ShirtGraphic") then
                v:Destroy()
            end
        end
	
	if game.ReplicatedStorage.MapName.Value == "Grassy Plains" then --//I just moved this if statement out of the loop
             randomclothes = game.ServerStorage.ZombieClothes.GrassyPlains:GetChildren()
             pickedclothes = randomclothes[math.random(1, #randomclothes)]
             for i,x in pairs(pickedclothes:GetChildren()) do
                 x:Clone().Parent = character
             end
         end
	
        wait(0.5)
        zombiespawns = workspace:WaitForChild("ZombieSpawns"):GetChildren()
        randomspawnn = zombiespawns[math.random(1, #zombiespawns)]
        character:MoveTo(randomspawnn.Position)
    end
    if game.ReplicatedStorage.RoundBeginning.Value == true then
        character.Humanoid.WalkSpeed = 0
    end
end

local function onPlayerAdded(player)
    player.CharacterAdded:Connect(onCharacterAdded)
end

Players.PlayerAdded:Connect(onPlayerAdded)