GitHub repository with your minimal reproducible example (do not fill out this field with "github.com/bufbuild/buf" or we will automatically close your issue, see the instructions above!)

https://github.com/stefanvanburen/buf-repros

Commands

cd ./lsp-duplicate-leading-comments
nvim test.proto
# format the file with the LSP

Output

Results in a duplicate leading comment, diff:

diff --git a/lsp-duplicate-leading-comments/test.proto b/lsp-duplicate-leading-comments/test.proto
index f383f8e..8e97aa1 100644
--- a/lsp-duplicate-leading-comments/test.proto
+++ b/lsp-duplicate-leading-comments/test.proto
@@ -1,4 +1,5 @@
 // Leading comment.
+// Leading comment.
 syntax = "proto3";
 
 package TODO;

Expected Output

No diff.

Anything else?

The issue seems to lie in using the fileNode in the range returned in the TextEdit; the AST doesn't count the leading comments when calculating NodeInfo.Start(), while the formatter touches the entire file.