didn't receive a ribbon

Then please send a support ticket about your problem.

There is a major problem with this specific Ribbon. You can easily find similar posts in the Forum about the same topic.
I had the same exact problem, but when I won a capsule shotgun and had 6 of them in my inventory, I received the Ribbon.

As I have come to understand, you can't get the Shotgun Ribbon with only purchasing GP shotguns from the shop.
I think that the Dual Desperados do not count as shotguns. You need to get at least one shotgun from capsules (either GR or ZP). You can also try to get the Serbu Super-shorty from the MP shop.