Does this actually work? I mean, why not say (600) instead? 
setSyncInterval(10 * 60); // Sync Blynk real time clock (RTC) interval in seconds (10 minutes)
I use a terminal.flush(); after my text blocks. But can’t remember if that helped for the duplicate string-error or not. Perhaps worth a try?!
Elsewhere in the forum it was suggested that it was due to lag caused by SimpleTimer. I always recommend to ease up on the timers where it’s possible because it seems to be the source of many strange errors.
Some other approaches is found in this thread.