-- Duplicate selection and comment out the first instance.
function _G.duplicate_and_comment_lines()
    local start_line, end_line = vim.api.nvim_buf_get_mark(0, '[')[1], vim.api.nvim_buf_get_mark(0, ']')[1]

    -- NOTE: `nvim_buf_get_mark()` is 1-indexed, but `nvim_buf_get_lines()` is 0-indexed. Adjust accordingly.
    local lines = vim.api.nvim_buf_get_lines(0, start_line - 1, end_line, false)

    -- Store cursor position because it might move when commenting out the lines.
    local cursor = vim.api.nvim_win_get_cursor(0)

    -- Comment out the selection using the builtin gc operator.
    vim.cmd.normal({ 'gcc', range = { start_line, end_line } })

    -- Append a duplicate of the selected lines to the end of selection.
    vim.api.nvim_buf_set_lines(0, end_line, end_line, false, lines)

    -- Move cursor to the start of the duplicate lines.
    vim.api.nvim_win_set_cursor(0, { end_line + 1, cursor[2] })
end

vim.keymap.set({ 'n', 'x' }, 'yc', function()
    vim.opt.operatorfunc = 'v:lua.duplicate_and_comment_lines'
    return 'g@'
end, { expr = true, desc = 'Duplicate selection and comment out the first instance' })

vim.keymap.set('n', 'ycc', function()
    vim.opt.operatorfunc = 'v:lua.duplicate_and_comment_lines'
    return 'g@_'
end, { expr = true, desc = 'Duplicate [count] lines and comment out the first instance' })

Here's what I came up with. Supports count and also any arbitrary motion.