There is code to not clobber existing signs for a line.

But

1. Neovim supports displaying multiple signs per line (e.g. set signcolumn=auto:3), so that should be optional

2. it uses incrementing IDs, so eventually might use an already existing ID from another plugin, and then would clobber that sign (regardless of the line it is being used in)

As for 2., AFAIK it is not necessary really to have different IDs per sign, i.e. gitgutter could just use a single ID (or maybe per sign type), as long as it unplaces all signs.