This question already has an answer here:
Case is following:
Someone has released software under ISC license, and the source code file has following section at the beginning
Copyright (c) 2015, 2018 John Doe
Permission to use, copy, modify, and distribute this software for any...
If some other party modifies some parts of this software and then releases it under ISC license, how should the copyright part be written? Should both names be mentioned?
The source code might not go to version control system, so it won't be trivial to figure out who wrote what parts.