I always regarded the Devlog section on this discussion board as an additional way to introduce people to your project and share updates with them, separate to the actual devlog feature which presents your updates to users via the dedicated search listing, as well as in the feed of people (following you). Those devlogs will also be added to your corresponding project page as sort of an info-pool and archive for future visitors. You can create multiple and update/archive/delete them individually, where as going from the rules in the devlog section on here, you should update one topic you created for your project with additional posts and edits.
Basically said: The 'real' devlogs are yours, they are part of managing your projects. The section on this discussion board is sharing your work progress with the community on here.
There are also two types of devlogs since a past update: You can post a devlog directly as part of your account, so it will not be bound directly to one of your projects. For this you can go into your Dashboard -> Posts -> Create a new post. They can basically seen as managing a 'blog'. Or, by choosing one of your projects, or creating a devlog while editing them, you can create devlogs tied to this specific project. You can use both for different context of writing about stuff more in general or specific, or some other reason you do not want to tie something to a specific project.
In regards to spamming: I would not think copying your devlog progress into your forum topic would be considered that, as well as vice versa.
If you intend to create actual devlogs, you should avoid copying the same information over and over again. A specific devlog does not need to be a general one as well. If you have something to write about multiple specific projects, a general devlog may be the better choice. The latter can be context-specific though, so if you have a logical reason, I doubt you would get in trouble for creating two similiar looking devlogs for a very rare occasion.
There are some other things about devlogs in how they are integrated and can be interacted with, but that should be the general gist of them.