As the manual gitignore(5) says "Files already tracked by Git are not
affected", this is not a problem as far as Git is concerned.
However, Git is not the only program that parses .gitignore because
.gitignore is useful to distinguish build artifacts from source files.
For example, tar(1) supports the --exclude-vcs-ignore option. As of
writing, this option does not work perfectly, but it intends to create
a tarball excluding files specified by .gitignore.