DaemonUmbra

You say this as if 1.7.10 is unique in this regard.

I don't have an answer for that currently. Worst case you can still use launcher profiles to target standalone Java installations.

It's not as simple as you think. Forge uses a lot of Java backend/internal stuff and most, if not all, of it has changed between Java versions 8 and 9/10. All of that has to be rewritten to work in Java 9+ It's being worked on though, Forge 1.13 will run on Java 9+

Are you using the standard Mojang launcher? Have you customized any of your launcher profiles? Where are you putting your mods? Please give the full path to your mods folder.

You have Java 10 installed and your computer is using that rather than Java 8. To launch with Java 8 instead of 10 you need to use the full path of the Java 8 java.exe, rather than just using the command java

You may have it installed, but that message means it's not the one being used. Open CMD and type java -version

You need to use Java 8.

1.7.10 is no longer supported on this forum due to its age. Please update to a modern version of Minecraft to receive support.

I'm not sure, but your computer will be more secure than it was with your firewall down.

Please don't hijack threads, especially when your problem is different. Make your own thread.

When I say "modern version" I mean 1.12.2 and that looks like you copy pasted from the console window, I wanted that actual debug.log file.

Please use Pastebin or Gist, I don't want to be forced to make an account somewhere to view a file.

Please post the debug.log of a modern version.

Some people refuse to give up their favorite mods from back in the day, they would rather live in the past and demand that mods be backported rather than find new favorite mods in modern versions Some modders are in that camp and others fall to peer pressure from that camp. There are also those who hated the combat update with a passion and refuse to accept the change.

Did you try restarting the router after port forwarding?

Then you should look up the instruction manual for your router, as each one tends to have its own organization scheme for the web panel.

Do not create clients. This is not negotiable or debatable, thread locked.

You have decided that you are fine with any and all signals, whether malicious or benign, doing whatever they want on/to your computer as long as your friends can access your server. Re-enable your firewall.

Port forwarding is not the same as allowing a program through your firewall, you should NOT take your firewall down completely for any reason. Port forwarding is telling your router that anything coming in on *this* port goes to *this* device on the local network.

Are you using Hamachi or do you have your router port-forwarded? Do you have your firewall set to allow Java through? Do you have an antivirus that might be considered to be overzealous?

Yes all of this is happening inside vanilla code. Is this only happening to one user? Are you using any mods that don't use Forge? (Server or Client) Are you using a hosting company or running on your own hardware?

I don't understand how that's possible unless a mod is doing something funky and re-triggering some of the login code or your user isn't reporting everything

So the message about logging in from another location is what they are being kicked with? Or is it saying that when they reconnect?

Do you mean they get kicked then kicked again on the rejoin?

The error shows up when a player logs in with a UUID that's already logged in. I suppose this could happen if the player loses connection and tries to reconnect before the server is even aware they disconnected.