diff --git a/lib/fetch-libc++.sh b/lib/fetch-libc++.sh
index b2bdc0fbf..b07b59e9a 100755
--- a/lib/fetch-libc++.sh
+++ b/lib/fetch-libc++.sh
@@ -49,7 +49,8 @@ done
 
 if [[ $FOUND -ne 0 ]]; then
   echo "Unpacking $ZIP_FILE..."
-  unzip -q $ZIP_FILE
+  # -n: never overwrite existing files, -q: quiet mode
+  unzip -n -q $ZIP_FILE
   echo "Done upacking $ZIP_FILE..."
 else
   echo "ERROR: Unable to find tock-libc++"
diff --git a/lib/fetch-newlib.sh b/lib/fetch-newlib.sh
index 1c26f4c08..cbbd3feba 100755
--- a/lib/fetch-newlib.sh
+++ b/lib/fetch-newlib.sh
@@ -47,7 +47,8 @@ done
 
 if [[ $FOUND -ne 0 ]]; then
   echo "Unpacking $ZIP_FILE..."
-  unzip -q $ZIP_FILE
+  # -n: never overwrite existing files, -q: quiet mode
+  unzip -n -q $ZIP_FILE
   echo "Done upacking $ZIP_FILE..."
 else
   echo "ERROR: Unable to find tock-newlib"