What? It's never occurred to me that this isn't entirely normal, I've done it all my life. I thought people without an inner monologue were the unusual ones.
Not so much imagining a conversation in your head, but playing through a conversation with "the other side" of the coin (be it an idea, plan, problem, solution etc) which are both yourself if that makes sense.
Is this not what the inner monologue to yourself is? Your inner is conversing with your outer monologue. Hard to explain.
Would consider myself to be both physically and mentally stable, no conditions etc.
I think it's okay, unless you converse online with your throwaways.