A few different scenarios where multiple accounts get created, such as reinstalling the app, new computer, new device, etc. Just to make sure this is not the case for you:
On the iOS app tap on More > Settings > Profile & Privacy > Manage Garmin Account. Note the Email address under the ACCOUNT DETAILS section of that page.
Go to Garmin Connect Web account information page. Verify that email matches the iOS app. Click on your device (top right corner) > device settings. Verify the serial number
Go to Garmin Express (if installed on your computer). Click on your device > tools & content > Info tab. Verify the "Registered to" matches the above email addresses and serial number.
