The Polymorph spell says:
This spell transforms a creature with at least 1 hit point that you can see within range into a new form. An unwilling creature must make a Wisdom saving throw to avoid the effect. A shapechanger automatically succeeds on this saving throw.
Changelings (from Wayfinder's Guide to Eberron), which are similar to doppelgangers, are able to change appearance, but don't seem to be explicitly shapechangers. Are changelings actual shapechangers and therefore immune to Polymorph?