Time Machine wants to start a new backup to hard disk

My Macbook just gave a message that it had completed a verification of the backup on an external hard disk and that it needed to start a new backup. I think the message also said this would overwrite the current file, which seems crazy.
I am making a duplicate of the old file before I let TM start the new backup since I don’t want to lose a couple of years of backups.
This is the first time I have encountered this situation. Did I misunderstand the message?
PS Running latest Mojave on a 2015 Macbook Retina

As far as I understand it, you did not misunderstand the message. I have experienced this repeatedly with Time Capsule. (To quantify that, I’ll guess that I’ve seen it eight times in four years, but irregularly. I believe I saw it twice within six weeks, but another time it was over a year between instances.) Interestingly, another user with backups on the same Time Capsule has never seen it.

For what it’s worth, my problems have been with a 2015 MacBook Retina.

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IME this isn’t a problem unique to you or your Mac or your OS version. TM has always been buggy and not terribly reliable, with issues such as yours or similar, but IMO it’s better than nothing. That means that if TM’s your only backup you’re walking on thin ice.

Actually I have 3 Time Machine backups of the Macbook on 3 separate drives (one is kept in my car). It is the one that is permanently connected that just got overwritten. The ability to backup to multiple drives is a major advantage of TM however I also make a Carbon Copy Clone backup every few months.
As foreshadowed, I created an “archive” folder and copied the TM sparsebundle file to that folder then allowed TM to overwrite the original File. That took 13 hours for 5.7GB.
I wondered if I could access the copied file using another computer but opening it with TM on that Mac is not an available option. I suppose that, if desperate, I could copy it back to the original folder and try opening it with the Macbook. I might also be able to view the package contents and access particular files - I might try this later.