golang dep add says "<path> is already imported or required, so -add is only valid with a constraint", what does it means?

Question

I'm trying to add a dependency to the vendor directory and I'm getting this really strange error:

dep ensure -add github.com/google/uuid

And the error says:

github.com/google/uuid is already imported or required, so -add is only valid with a constraint

At some point, while writing my code, I did:

git get github.com/google/uuid

and now I'd like to add uuid to my vendor/... directory.

But as far as I know it should have no side effect, correct?