If Tools/Options.../C++/Clangd option is On we have next trouble:
When using vertical document split (side by side) top level outline of document is often (almost always) shows function declaration twice.

To avoid this problem I do next: (see attached screenshots)
1. open Tools/Options.../C++/Clangd option;
2. deselect "Use clangd";
3. press "Apply";
4. select again "Use clangd";
5. press "OK";
6. And continue to development.

I have to do it almost always when I switch to another source/header file either on left editor pane or right!