Sitemap "Found duplicate sitemap name"

The log shows:
“sitemap.internal.SitemapResource] - Found duplicate sitemap name ‘_default’ - ignoring it. Please check your configuration.”

I have no idea, what the reason for this may be. There’s only one site file with the first line: “sitemap _default label=”…
Guess, the message is misleading and something else is duplicated.

Did you manually create this sitemap?

Yes - created over several years

A _default sitemap is automatically created my the system!

how delete this?
Or should I rename mine?

The system can’t be deleted, so you’ll have to rename yours.

Thanks for your help