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
 
				 
		 
					 
				