Context: I'm riding a Peloton bike indoors and I use a Garmin watch. To sync the performance data of my Peloton (power meter), I use https://syncmyworkout.com.

Problem: My watch recording posts to Garmin Connect and this immediately gets sent to TrainingPeaks. Then later SyncMyWorkout merges that watch recording with the Peloton workout, uploads the merged file to Garmin (which is then sent to TrainingPeaks) and then deletes the original watch recording in Garmin. During this process two workouts have been sent onwards to TrainingPeaks and thereby creating a double entry in Training Peaks.

Solution: The developers at Intervals.icu have cleverly coded things so that if something gets pushed to Intervals.icu from Garmin, and then is later deleted from Garmin, Intervals.icu can detect this and deletes it from Intervals.icu (effectively mirroring what happened in Garmin). Ideally TrainingPeaks would have set up their Garmin integration that way.

FYI: The team from SyncMyWorkout applied for TrainingPeaks API access a few weeks ago to see if they could build a direct integration but says you never responded to them. Maybe working with them might solve the issue quickly... ;)