diff --git a/add_extension.sh b/add_extension.sh
index d5e69c5..245d114 100644
--- a/add_extension.sh
+++ b/add_extension.sh
@@ -3,7 +3,18 @@
 EXT_NAME=$1
 EXT_VERSION=$2
 
-cd /app/extensions
-git clone https://gerrit.wikimedia.org/r/mediawiki/extensions/$1
-cd $1
-git checkout -b $2 origin/$2
+cd "app/extensions"
+
+if [[ -e "${EXT_NAME}" ]]; then
+    cp -Rv "${EXT_NAME}" .
+    cd "$(basename ${EXT_NAME})"
+else
+    git clone "https://gerrit.wikimedia.org/r/mediawiki/extensions/${EXT_NAME}"
+    cd "${EXT_NAME}"
+fi
+
+if [[ "${EXT_VERSION}" ]]; then
+    git checkout -b "${EXT_VERSION}" "origin/${EXT_VERSION}"
+fi
+
+composer install --no-dev