Interesting - I deleted both entries, and afterwards there were still two entries, though the times had changed: both were now 90 minutes earlier than previously shown. So from this I surmise that there were in fact duplicate entries for both backups, but crontrol was only showing the second (later) entry for each duplicate.
At this point I expect that the jobs will still run, but just once per day. We'll see how it goes. I'll mark this resolved in a couple of days if all is well.