So your keyboard has a left Alt key and a right Alt Gr key needed for symbols, and using the right Alt Gr key isn't an option because you already use it for something else. BTW, the US keyboard has two Alt keys and no Alt Gr, and either Alt key in conjunction with a Click will set the foreground color, which is why I answered the way I did, initially.
If you temporarily change the Duplicate Layer to another key besides Alt Gr does Alt Gr+Click select the foreground color just like Alt-Click?
The reason I ask is because I don't think you can substitute a non-modifier key (like P or ;) for a modifier key like Alt, Shift, Ctrl, Alt Gr, so redefining what you have the Alt Gr key assigned to might be the only way, then you can decide if that's what you want to do or not.
When the paint-tool is active, what happens when you press the Alt key is the cursor changes to the eyedropper and you can move the eyedropper anywhere and then click to sample the color at the cursor position. I'm pretty sure that can only happen with one of the modifier keys not any key, so pressing the P key continuously isn't going to show the eyedropper, for example, and I don't see any way to change the Alt keyboard assignment in PS-Paint mode, only PS, outside of any tool.