[hlieberman]

That's correct only if the implementation was made without using the original implementation - for example, made off of a common specification, or using only the inputs and outputs of the application. (AKA, a "clean room" implementation)

If the reimplementation was made by perusing the original, it can be considered a derivative work subject to copyright.

[gorbachev]

How would you ever prove "perusing the original" conclusively?

Sure you could say "so and so downloaded Monit using IP address of 127.0.0.1 at this time", but I don't think that really is legally conclusive.

Unless the alleged violator actually posted public comments about "perusing" the original source code there's not a lot you could do. Unless you do discovery on all of his computers and he didn't delete the Monit source code. To go through all that trouble for a copyright violation on free, open source software seems hardly worth it.

[danielweber]

Things are seldom "proven" in court the way they are in mathematics. Instead, Mike would swear that he had never looked at the source code, which, if his lawyer is any good, he will only let him do if it's actually true, because it would open him up to perjury charges. Later, if Monit had evidence this wasn't true, they would present it. IANAL.

[IamThePherocity]

The clean room was argued by Google to aid with the justification, but this isn't a requirement of law. An author can read another authors work, and create a novel with similarities.