golang dep add说“ <路径>已经被导入或需要，所以-add仅在约束条件下有效”，这意味着什么？

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

Now, I did:

git get github.com/google/uuid

while writing my code 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?