Whilst the 174 will have ADS-B it's highly highly unlikely to have HFDL....
I wonder if by any chance the OP's observations are anything at all to do with this (from Planefinders website):
Federal Aviation Administration (FAA Feed)
Not all aircraft are fully ADS-B equipped. In locations such as Europe and Australia the implementation of ADS-B is well advanced and the majority of aircraft use ADS-B. North America is behind currently but is catch up as the US NextGen ADS-B equipage project continues to develop up to, and beyond the 2020 mandate.
Where we do not have ADS-B or MLAT coverage for an aircraft in North America we will supplement this using a 5 minute delayed feed from the FAA. These are the orange planes in Plane Finder (My emphasis).
The FAA feed covers US and Canadian airspace including bordering areas of the Atlantic and Pacific Oceans.
Wonder when/where the OP is seeing the two "returns"?
Is it possible that when the 174 is on the edge of Planefinders own ADS-B coverage the OP is seeing displayed at times both the real time ADS-B position and the FAA 5 minute delayed position - I wonder if there's 40 miles'ish separation when two returns are displayed?
Before anybody asks, yes, if it happens to the 174 I'd assume it would happen with other similarly equipped flights in similar geographic positions.