I found these in my dykbot-cron.err file.  What causes this?  Is it something to worry about?

WARNING: API error mwoauth-invalid-authorization: The authorization headers in your request are not valid: Nonce already used: <hex string elided>

ERROR: Retrying failed OAuth authentication for wikipedia:en: The authorization headers in your request are not valid: Nonce already used: <hex string elided>