When using `NERDTreeMirror` and `mksession` you get error `Buffer with this name already exists`

[giggio]

Self-Diagnosis

Before creating an issue, take some time to search these resources for an answer. It's possible that someone else has already seen and solved your issue.

• old NERDTree issues
• NERDTree documentation - :h NERDTree
• NERDTree Wiki
• Other resources: https://stackoverflow.com, https://vi.stackexchange.com, etc.

Environment

• Operating System: Ubuntu 24.04.2
• Vim/Neovim version :version: VIM - Vi IMproved 9.1 | Included patches: 1-1336
• NERDTree version, found on first line of quickhelp ?: 7.1.3
• Are you using any of these NERDTree-dependent plugins?
  • Xuyuanp/nerdtree-git-plugin
  • ryanoasis/vim-devicons
  • tiagofumo/vim-nerdtree-syntax-highlight
  • scrooloose/nerdtree-project-plugin
  • PhilRunninger/nerdtree-buffer-ops
  • PhilRunninger/nerdtree-visual-selection
  • jistr/vim-nerdtree-tabs
  • Others (specify):
• Provide a minimal .vimrc file that will reproduce the issue.

let s:test = expand('~/p/vimbug')
let &runtimepath = s:test . ',' . &runtimepath
let &packpath    = &runtimepath

function! ConfigureNERDTree()
  if exists("g:NERDTree")
    noremap <F2> :NERDTreeMirror<CR>:NERDTreeToggle<CR>
    " Open the existing NERDTree on each new tab.
    autocmd BufWinEnter * if &buftype != 'quickfix' && getcmdwintype() == '' | silent NERDTreeMirror | endif
  endif
endfunction
autocmd VimEnter * call ConfigureNERDTree()

Steps to Reproduce the Issue

1. Create the provided .vimrc at ~/p/vimbug
2. Run vim -u ~/p/vimbug/.vimrc -N -n
3. Create 2 files, open each one in a tab. NERDTree buffer will be mirrored
4. Run command :mksession!
5. Quit vim
6. Start vim again, this time with -S, so the session is loaded: vim -u ~/p/vimbug/.vimrc -N -n -S
7. Error will show
8. Inspect Session.vim, you will notice two calls to file NERD_tree_tab_1, the error is happening at the second one

Current Behavior (Include screenshots where appropriate.)

Errors when session is loaded.

Expected Result

Session loads as expected without error.