As long as the completions have the same annotation (the text in green) and same kind (the icon on the left), they are deduplicated.
Looks like perhaps lsp-mode generates annotations that are more useful for your scenario than the ones that eglot does. If that is the case (and not, maybe, that you have configured lsp-mode/eglot to use different language servers), I suggest filing an issue. This shouldn't take too much time to change.
Something like that would be generally easy to see very soon (completion popup without both icons and annotations). This approach has worked for years for both Emacs's default UI and company-mode, so it's hard to call it fragile.
I suppose it might have been a cause for investigation for some backend authors at some point, but backends would generally avoid internal copying anyway, for performance reasons if nothing else.