check that desired release tag does not exist

.github/scripts/check_tag_does_not_exist.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+# This script checks that the TAG arg does not exist, already.
+
+TAG="$1"
+
+if [ $(git tag -l $TAG) ]; then
+	echo "Error; tag $TAG already exists"
+	exit 1
+else
+	echo "tag $TAG does not exist"
+fi

.github/workflows/create-release.yml

@@ -24,6 +24,9 @@ jobs:
       - name: Verify that the current branch has a name that starts with 'release-'
         run: .github/scripts/verify_is_on_release_branch.sh
 
+      - name: Verify that the desired release tag does not already exist
+        run: .github/scripts/check_tag_does_not_exist.sh
+
   create-draft:
     name: Create draft release
     needs: verify-head-status