So I own a two year old Instinct, and recently added a new FR 55. I've Connect on an Android Pixel 2. Express on a Win10 PC.
A few days back, after wearing my new FR 55 for several days, I recorded an Activity with my Instinct (to let my new FR 55 charge) and it duplicated after syncing with Express. I returned to wearing my charged FR 55.
No HR info was recorded at all, yesterday, or this morning, I just noticed this morning.
Yesterday I recorded another activity with my FR 55. It duplicated after syncing with Express this morning. After syncing with Express, I got HR info recording again.
I also earned a badge I don't deserve, as a result of the duplicate activities. I _hate_ that there seems to be no way to undo that :-(
None of this was happening before adding my FR 55 to my account (which I set as my preferred tracker, since it is nicer to wear while sleeping). So that's why I suspect the FR 55. But I just don't know.