Does this mean that I have multiple copies of GRUB taking up space?
No, grub installs most of itself into the root filesystem partition (see subdirectories of
/boot), so if by "installing multiple times" you mean overwriting one install after another, that's all overwritten too. EFI/GPT systems use some extra hidden space, but that will also always get overwritten (there won't be more than one such space on the drive).
Just to be clear: if you only have one existing linux on the machine, stop reading now.
However, if you mean you have multiple parallel installs on separate existing partitions, then they do all have separate copies, even though only the last one installed is used (make sure you are certain about which one, obviously). It is probably okay to delete the unused ones. An easy test would be to move the
efi subdirectories in
/boot temporarily to a separate partition and then try to boot that system.
how should I safely remove the excess copies?
Use the distro package manager, there should be nothing that depends on it although it may be in separate pieces (grub, grub-efi, grub-tools, grub-themes). If you don't use the package manager, it may end up coming back as an upgrade later. Keep in mind the total size is at most only a dozen MB or so and generally not worth bothering about. Also, I can't promise that some package managers won't do something stupid with the boot sector of the drive (although that would be pretty stupid indeed).