Been there, done that. Looks as if whoever did it first did not get good penetration, probably trying to weld with the door and fender still on.
Do as dj99 suggests if at all possible. Get a piece of reinforcement behind the hole, even if you can only tack it, and then weld again, making sure you get a good deep, well formed weld. MIG works well for this.
The original attachment, for reasons I've never understood, is welded only on top, bottom and front edge, leaving the rear edge open. It tears out from rear forward, of course. Last time I did this, after I had the hinge well placed and welded in, I put a piece of 3/16 or so rod into the rear edge, welded over it, to provide a wide bridge of reinforcement. It never flexed again.
Especially if this is a 2-door, you'll find that the hinges are always stressed, not just by the weight of the door, but because when you open the door, it swings too far out. The door check begins to pull out on the frame of the door, and make it worse. The hinges themselves have a stop built in, and when the door passes the usual check point and hits the stop in the hinge, it pries the hinge right out of the vehicle frame like a sardine can key. Bad design there.
Once you have the hinge rewelded, you can help prevent further problems by making sure that the door itself is not distorted where the check bolts in, and if you're handy at welding, weld the hole in the end of the check shut, and redrill it about !/4 inch further back. Grind to fit and reinstall. The door will not open quite so wide now, and will stop swinging before the hinges hit their limits. You can also grind out the limits on the hinges a little.
