diff --git a/src/builder/b-mirror.c b/src/builder/b-mirror.c
index 5e4e768..6822da4 100644
--- a/src/builder/b-mirror.c
+++ b/src/builder/b-mirror.c
@@ -43,6 +43,16 @@ static const char * const git_helper_sh =
 	"    REF=$2\n"
 	"    HASH=$3\n"
 	"    MIRROR_PATH=$4\n"
+	"    while ! mkdir \"$MIRROR_PATH.lock\" 2>/dev/null; do\n"
+	"        echo \"git mirror locked, waiting...\"\n"
+	"	 sleep 1\n"
+	"    done\n"
+	"    trap 'rm -rf \"$MIRROR_PATH.lock\"' EXIT\n"
+	"    if [ -d \"$MIRROR_PATH/.git\" ]; then\n"
+	"        if git -C \"$MIRROR_PATH\" rev-parse -q --verify \"ref-$HASH\" > /dev/null; then\n"
+	"            exit 0\n"
+	"        fi\n"
+	"    fi\n"
 	"    mkdir -p \"$MIRROR_PATH\"\n"
 	"    if [ ! -d \"$MIRROR_PATH/.git\" ]; then\n"
 	"        git init --bare \"$MIRROR_PATH\"\n"
@@ -85,20 +95,39 @@ static const char * const git_helper_bat =
        "    echo \"REF: !REF!\"\n"
        "    echo \"HASH: !HASH!\"\n"
        "    echo \"MIRROR_PATH: !MIRROR_PATH!\"\n"
+	"    :lock_wait\n"
+	"    mkdir \"!MIRROR_PATH!.lock\" 2>nul\n"
+	"    if errorlevel 1 (\n"
+	"        echo \"git mirror locked, waiting...\"\n"
+	"        timeout /t 1 /nobreak > nul\n"
+	"        goto :lock_wait\n"
+	"    )\n"
+	"    if exist \"!MIRROR_PATH!\\.git\" (\n"
+	"        git -C \"!MIRROR_PATH!\" rev-parse -q --verify \"ref-!HASH!\" > nul 2> nul\n"
+	"        if exist !MIRROR_PATH!\\.git if not errorlevel 1 (\n"
+	"            rmdir \"!MIRROR_PATH!.lock\"\n"
+	"            exit /b 0\n"
+	"        )\n"
+	"    )\n"
 	"    if not exist \"!MIRROR_PATH!\\.\" (\n"
 	"    mkdir \"!MIRROR_PATH!\"\n"
 	"    )\n"
 	"    if not exist \"!MIRROR_PATH!\\.git\" (\n"
 	"        git init --bare \"!MIRROR_PATH!\"\n"
-	"        if errorlevel 1 exit /b 1\n"
+	"        if errorlevel 1 (\n"
+	"            rmdir \"!MIRROR_PATH!.lock\"\n"
+	"            exit /b 1\n"
+	"        )\n"
 	"    )\n"
 	"    set \"REFSPEC=!REF!:ref-!HASH!\"\n"
        "    echo \"REFSPEC: !REFSPEC!\"\n"
 	"    git -C \"!MIRROR_PATH!\" fetch \"!REMOTE_URL!\" \"!REFSPEC!\" 2>&1\n"
 	"    if !ERRORLEVEL! neq 0 (\n"
 	"        echo \"git fetch failed with errorlevel !ERRORLEVEL!\"\n"
+	"        rmdir \"!MIRROR_PATH!.lock\"\n"
 	"        exit /b 1\n"
 	"    )\n"
+	"    rmdir \"!MIRROR_PATH!.lock\"\n"
 	"    exit /b 0\n"
 	")\n"
 	"if /i \"!OPERATION!\"==\"checkout\" (\n"