As posted above, it happens, and it will happen again. Also, as posted, slow down and push the open door side into the wind with a little opposite pedal- if it's convenient to "rassle" with the door in flight. Heavy emphasis on the convenience factor. If inconvenient- land and secure the hatch. If that's not possible immediately, slow down and continue to a "land as soon as soon as possible" point, land, and secure the hatch.
It's exciting when the hatch pops open of it's own accord- it has, does and it will happen. The immediate danger is unsecured objects, especially the light and durable stuff that exits and could be a rotor hazard.
Next, anything with ballistic potential that could endanger those below. If you secure loose objects, as we all do, it's not an issue (clearing throat)- the "falling object/part of the airframe lost in flight" paperwork IS a serious issue.
Getting excited at the change in noise level, and reacting in that mode, is your next most immediate hazard. If the door hasn't parted company with the helo at cruise speed, it *probably* won't and will continue as part of the airframe. That's not necessarily a hazard- usually the hatch opens a few inches and oscillates a bit. I don't know what kind of forces are acting on the unsecured door but my experience is they already have at this point. Losing control of the aircraft is a definite hazard, avoid it. If it's your door and you can secure it quickly and cleanly without intricate maneuvers, do so. Otherwise land, void, secure the hatch, and continue. Landing is easy, we've all done it many times.
If there's a "critter" sitting by the door, see if it's coherent with all the adrenaline coursing thru it's body. If so, consider -carefully- having the critter- er, uh- the "passenger" close the door. Danger- the critter's eyes will big as saucers and each shall have the strength of ten ordinary mortals, yea, verily! You want to cool them down, or you don't want them involved with a fragile and expensive part of the aircraft. I have seen perfectly sound doors exposed to excited rig hands done serious damage. If everybody isn't breathing normally- once the door closing starts, it's a self-exciting exercise and difficult to control, and pretty soon butts are unbelted and scrambling around, and the potential for spontaneous sky diving enters the picture. Tis is also a serious paperwork issue, and grounds for termination.
To recap, it's a common occurence. Slow down, secure the hatch, or land and secure it. Don't, repeat don't make it an emergency.
P.S. I wouldn't screw with a partially secured door in flight. Let sleeping dogs lie.
Fly the aircraft.