I have a vivoactive 4 and am using Garmin connect on iOS, though I believe I have seen this happen with my old vivoactive 3 and on the android platform.
The issue is that if I don't sync every single day, then when I finally do sync - the latest full day recorded from previous sync will be duplicated for the following day. (calorie count, including active and resting calories will duplicate; It does not duplicate the step count; that remains distinct for each day)
As an example, lets say I sync Tuesday morning, and then don't sync again until Saturday morning. It will look like the table below (day/steps/calories).
Tuesday's calorie data will very often just copy Monday's calorie data, though the steps/sleep/etc will all be distinct.
Monday 8500 2500
Tuesday 9200 2500
Wednesday 6200 2350
Thurs 7500 2410
Friday 8200 2450