That's a pretty good idea. I initially didn't want to do this, because that would also mean that other team mates could take over your IDE key, as they can just repoint it to their IDE. I think I will have to make this an optional flag.

I agree with the previous suggestion, it would help with the annoying case of thinking you're all set up to debug a hard to reproduce bug only to find out the connection failed because of this. :)

Another, somewhat related, suggestion would be for the proxy to have a timeout and let execution continue if the registered client does does not respond within that time.
We're often running into a near catch 22 where we've forgotten to disable the little Xdebug helper extension for the browser before navigating away from a site we've been debugging and closing the IDE.
The next time you try to visit the page the cookie gets passed along, Xdebug connects to the proxy, which waits for the IDE, and then nothing more happens. Once you realize it's waiting for a debugger you have to either restart the proxy or start the IDE and unregister your key. The helper extension could be a bit more helpful here of course, as right now you can't stop it from sending along the cookie without actually having visited the site first or unloading it completely.
The IDE could of course also be improved to automatically unregister the client before closing. Having a timeout in the proxy would be very convenient if the other projects can't compensate for this situation.

I have just updated a new version of dbgpProxy (0.5.0), and you can find it at https://xdebug.org/download#dbgpProxy

It has the following changes:

1. There is a new -f flag. If set, IDEs can register with a key, even if that key has already been registered. It will replace it.
2. I fixed a bug where the proxy would sit in an unresponsive state for a key, if it could not set up a forwarder as the IDE could not be connected to.
3. I fixed another bug where the connection would hang when removing a key registration without an actual connection

The 1st point should address the original issue here.

The 2nd point should address "suggestion would be for the proxy to have a timeout and let execution continue if the registered client does does not respond within that time."

0.5.0 was just what we needed, it is going to make things much smoother for us, especially when moving between the office and home machines.
I can confirm both the -f flag is working and the site no longer hangs if the xdebug helper was left enabled. I think I may have seen 3 happen earlier, but thought it was an IDE bug. I can not provoke it to happen anymore so that's an added bonus.
Thank you!