Garmin Drive 51 Duplicates Routes sent from Basecamp

I create a set of Routes in Basecamp and transfer them to my Garmin Drive 51. 

I disconnect the Garmin Drive 51 and all the Routes appear in Trip Planner as expected.

Later, after I have created new Routes in Basecamp and connect the Garmin for transfer, I find the earlier transferred routes have been duplicated..in most cases a "1" has been added to the Route name. 

Am I doing something to cause this duplication? Any way to avoid it?

I'm running Basecamp 4.8.11 on Mojave 10.14.6.