What you have reported, often happens to new users of Pencil2D. The way that Pencil2D works, when you save a image, is that it adds, the default extension.
If you type the your desired filename, with an extention, Pencil2D doesn’t understand your mistake! And it adds a second extension.
To solvecthe problem, use the file rename function, within your operating system to correct the filename, by removing the 2nd .extension.
Yes in one respect, your right, OK it would be a nice to have modification, but an understanding of saving files and file naming conventions, you would have deleted the ‘.png’ from the suggested filename, clicked on the button and the save would have worked!
I suppose what I’m saying, I’m a traditionally trained artist. I was taught, if I hit a problem, it was duty to find a solution myself and not to rely on others to find it. It’s called a work around!
@FraFraFrankenstone I’ll check this and report accordingly to the devs. But for now you can literally just delete the format in the name prompt and it will not add it twice.