Here are my thoughts regarding this,
Find the root cause - why the developer is estimating wrong?
Try to fix the root cause i.e. collaborate and train the developer on how to estimate correctly. [Use the other answers like 4x, 2x estimations as needed]
Long Term Fix
The idea of creating a self-organizing team.
Usually in agile, Developers are ...
You have to multiply programmer estimates by at least twice. A day will turn into two days. An estimate of two days will turn into four days etc.
However anybody who constantly fail your x2 estimation should not be in the team. You will need to let them go. It’s part of project management hence your job to prepare for job termination if the estimates can’t ...