We recently saw issues regarding duplicate USPS tracking numbers. In this particular case our customer was using Stamps.com so we contacted Stamps.com and here is their response:
What we have observed recently from the USPS website is USPS has not been scrubbing out old tracking information from their tracking events page. Once the USPS receives our daily report of available tracking numbers we issued from the tracking number pool, the USPS website then updates that webpage to reflect current tracking event information.
Please note that tracking numbers older than 6 months are recycled and are made available for use by the USPS.
We are actively reaching out to the USPS to inform and make them aware of this behavior and how it's impacting customers. We'll be sure to follow-up with additional information as it becomes available.
We will update this post accordingly.