People might not internalize their achievements, causing them to feel like they did not earn those. This makes them feel a constant fear of being exposed as a "fraud". This is very common, and I have no idea how to avoid it (either for myself or for others).
Stanier (2020) suggests in its 10th chapter that pairing different level people might show them how far they've come. The author also suggests reinforcing good achievements for these people.
Coaching and Mentoring might also be good tools to deal with that.
Source:
(Impostor syndrome, 2021)
2026-10: In a sense, maturity fixes a lot of it. It doesn't go away, but consistent reinforcement (for others and oneself) greatly helps. For the personal side (you trying to convince yourself), speaking as if you were another person helps me spot points where I'm being excessively harsh.