snap autobuilds are not inhibited by a manual build

[Andy Whitcroft]

Manual builds of snaps don't inhibit auto builds, even if they were created with the very same parameters and the same set of architectures.

This leads to duplicate uploads of packages to the store.

[William Grant (wgrant)]

I'm not sure we can sensibly avoid this. The resources included by snapcraft.yaml may have changed, and it's somewhere between difficult and impossible to detect that.