guard against duplicate tags

src/scripts/release.sh

@@ -5,6 +5,14 @@ set -e
 TAG=$(python -c 'from olmo_core.version import VERSION; print("v" + VERSION)')
 
 git pull
+
+# Make sure tag/release doesn't already exist.
+STATUS_CODE=$(curl -s -o /dev/null -w "%{http_code}" "https://github.com/allenai/OLMo-core/releases/tag/${TAG}")
+if [[ $STATUS_CODE == "200" ]]; then
+    echo "Release tag ${TAG} already exists"
+    exit 1
+fi
+
 python src/scripts/prepare_changelog.py
 
 read -rp "Creating new release for $TAG. Do you want to continue? [Y/n] " prompt