Garmin Connect says "duplicate activity" after an edit

You trimmed a ride, fixed the heart rate, or merged two files, uploaded the result, and Garmin Connect refused it as a duplicate. Strava does the same, with a message about the activity already existing. Both are behaving as designed, and the fix is a matter of order.

Why the platform thinks it's a duplicate

Garmin Connect and Strava check a new upload against the activities you already have. The main signals are the start time and the device it came from. An edited file keeps the same start time (unless you cut the start off) and the same device, so it looks like the same activity uploaded twice, which is exactly what the check is there to catch.

Cutting the start moves the start time, which is why a file with the beginning trimmed sometimes slips through while a file with the end trimmed doesn't. It is not worth relying on.

The order that works

Delete the original activity first. On Garmin Connect, open the activity, click the gear, and choose Delete. On Strava, use the three-dot menu. Do this on every platform you upload to. If Garmin syncs to Strava automatically, deleting on Garmin Connect doesn't delete the Strava copy, so delete there too.
Then upload the edited file. With nothing to match against, it goes through.
If Garmin auto-syncs to Strava, the edited activity should sync across after a few minutes. If it doesn't, upload the file to Strava directly.

If you already uploaded and it was still rejected

Wait a minute after deleting and try again. The deletion can take a moment to register. If it fails a second time, check for a second copy of the original: a manual upload plus an auto-sync from the device often leaves two.

Which editors cause more duplicates

Some older tools write the edited data back under the original file header, without recalculating anything. Garmin Connect sometimes flags those even after the original is deleted. FIT File Editor writes a fresh file with recalculated totals, which avoids that, but the start time is still the start time, so the delete-first rule stands.

Keeping the original

Deleting the activity on Garmin Connect doesn't delete the file on the device or the copy you exported. If you want the unedited version for reference, keep the exported original somewhere before you delete.