Implementing the same idea under GPL and under MIT

Question

What are the limitations and constraints if one wants to implement the same idea in different languages requiring different licenses?

To make it more concrete, say some algorithm should be implemented in R (under the GPL3 license) and Python (under the MIT license).

I'm in particular interested in the following scenarios:

1. R and Python package are developed jointly
2. R package already exists, Python package is developed later
3. Python package already exists, R package is developed later

In cases 2. and 3. the the goal is to re-use the existing content

My presumptions are:

• 1. is not problematic at all
• 2. is problematic because the Python package would be derived work and thus not compatible with a license change to MIT
• 3. is not problematic because the MIT license is permissive

Questions:

• Are my presumptions correct?
• What is the best way to re-implement / port GPL code to MIT with re-using the already existing content? Is this possible at all, or should does everything be written from scratch?

Answer 1

If you are the sole copyright holder on the code, you can do anything you like with it. It doesn't matter if you've published it under the GPL, you are always free to dual-license it under the MIT license, as proprietary code or any other license you feel like.

In legal terms, you publishing code under a license grants additional rights to people other than you in how they use the code. It it no way restricts what you can do with the code.