How to check if player has two of the same tool in Backpack?

Iterating through the backpack and keeping tally should do fine for this.

local backpack = game.Players.LocalPlayer.Backpack

local function getToolCount(toolName)
    local toolCount = 0
    for i, child in pairs(backpack:GetChildren()) do
        if child:IsA("Tool") and child.Name == toolName then
            toolCount = toolCount + 1
        end
    end
    return toolCount
end

-- eg: local swordsInBackack = getToolCount("Sword")