To reproduce, navigate by Tab key, or VO + left or right arrow keys.
"My Test 3" label will be double announced (see tests below) by VoiceOver with Safari 12 on macOS High Sierra 10.13.6 (17G65).
Announced as:
My Test 1, unchecked, checkbox.
Announced as:
My Test 2, unchecked, checkbox.
Announced as:
My Test 3, My Test 3, unchecked, checkbox.
Pairing Chrome with VoiceOver does not reproduce this issue.
Reviewing the Accessibility information in the Safari dev tools Node tab, the label of the checkbox is surfaced as a singular instance of "My Test 3".