diff --git a/cn/dev-mirrors.json b/cn/dev-mirrors.json
index b172483e..50f14221 100644
--- a/cn/dev-mirrors.json
+++ b/cn/dev-mirrors.json
@@ -9,7 +9,7 @@
   },
   "cargo": {
     "china": "sparse+https://rsproxy.cn/index/",
-    "official": "sparse+https://index.crates.io/"
+    "official": ""
   },
   "go": {
     "china": "https://goproxy.cn,direct",
diff --git a/cn/lib/dev-mirror.sh b/cn/lib/dev-mirror.sh
index f15889fc..37bb981e 100644
--- a/cn/lib/dev-mirror.sh
+++ b/cn/lib/dev-mirror.sh
@@ -26,11 +26,9 @@ cn_dm_set_line() {
 
   mkdir -p "${file%/*}"
   touch "$file"
-  if grep -qE "$match" "$file"; then
-    sed -i -E "s|$match.*|$line|" "$file"
-  else
-    echo "$line" >> "$file"
-  fi
+  grep -vE "$match" "$file" > "$file.omarchycn-tmp" || true
+  printf '%s\n' "$line" >> "$file.omarchycn-tmp"
+  mv "$file.omarchycn-tmp" "$file"
 }
 
 cn_dm_get_npm() { grep -sE '^registry=' "$HOME/.npmrc" | cut -d= -f2- || true; }
@@ -71,8 +69,14 @@ cn_dm_set_cargo() {
   mkdir -p "${f%/*}"
   touch "$f"
   sed -i '/^# OmarchyCN dev-mirror begin$/,/^# OmarchyCN dev-mirror end$/d' "$f"
-  if [[ -n $url ]]; then
-    cat >> "$f" <<EOF
+  if [[ -z $url ]]; then
+    return 0
+  fi
+  if grep -q '^\[source\.crates-io\]' "$f"; then
+    echo "cargo: $f already defines [source.crates-io]; edit it manually" >&2
+    return 1
+  fi
+  cat >> "$f" <<EOF
 # OmarchyCN dev-mirror begin
 [source.crates-io]
 replace-with = "omarchycn"
@@ -81,7 +85,6 @@ replace-with = "omarchycn"
 registry = "$url"
 # OmarchyCN dev-mirror end
 EOF
-  fi
 }
 
 cn_dm_get_go() { grep -sE '^GOPROXY=' "$HOME/.config/go/env" | cut -d= -f2- || true; }
@@ -90,7 +93,14 @@ cn_dm_set_go() { cn_dm_set_line "$HOME/.config/go/env" '^GOPROXY=' "GOPROXY=$1";
 cn_dm_get_gem() { grep -sE '^- ' "$HOME/.gemrc" | head -1 | sed 's/^- //' || true; }
 
 cn_dm_set_gem() {
-  cat > "$HOME/.gemrc" <<EOF
+  local f="$HOME/.gemrc"
+
+  if [[ -s $f ]] && ! grep -q '^# OmarchyCN dev-mirror managed$' "$f"; then
+    echo "gem: $f has existing user content; set :sources: manually" >&2
+    return 1
+  fi
+  cat > "$f" <<EOF
+# OmarchyCN dev-mirror managed
 ---
 :sources:
 - $1