GIMP Version: 2.6
Operating System: Windows
GIMP Experience: Advanced Level

List any relevant plug-ins or scripts:
N/A

List any ERROR messages you received:
N/A

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This may have been asked before, but I couldn't find anything about.
I'm looking to make a 'brightness/contrast' adjustment that will effect all visible layers simultaneously.
Is there any way of doing this?

Try this one Wallace.
It's just one of a suite of filters, so you will find it under Filters->Astronomy.

Thanks FB, I didn't think to check 'Astronomy' so I my just have this script. I downloaded it anyway just in case.

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I did have the script you suggested and this should work, but I'll have to duplicate the image and remove any layers I don't want to apply the brightness and contrast to.

Thanks again FB for your suggestion, I would've never thought to look in 'Filters->Astronomy' and I don't think I would have stumbled across it either.

Sorry for the delay in responding, I missed your edit.
I don't follow you, just make them non visible and check the option as shown in the script dialog below.

Maybe you have a different version than the one I attached?

Thanks FB and your right, I was mistaken. Didn't notice the check box for 'Visible' in the dialog.
Anyway... your suggestion worked like a charm and I was able to do exactly what I wanted, with great results.

BTW... I was working on a depth-map of an image and needed to adjust more then a few layers at a time.
It's unfortunate that there's no preview image for this script.