Maybe the Summoning won't be done by his name but by blood. -Harry is different from Harry, they don't have the same Name any more. It's the nature of mortal Names to change over time. So -Harry can't know it. But they do share the same blood.
Or -Harry just tries out a couple of variants of his name in the hope that one will find another Harry that is still alive.
I imagine the whole reason !Harry can summon Harry is he does know his full name. He just has to slightly modify the idea behind it, maybe the precise inflections, with the intention to summon a slightly off version of yourself.
Harry does precisely the same thing to Elaines name in WN btw so it's not entirely unprecedented. Little Wag: I think this is why !Harry is going to get much more than he bargained for, he's imagining a Harry already deceased(Harrys cup broke the lid but it's still full, if you know what I mean). Which is a fallacy of perception towards life and death on !Harry's part I think.
*Harry being good at deducing names may be another OG Merlin link, He had to have some way to get all those things there after all.
@Quantus, #2Don't we have a woj about if what happened in TC violated any any laws with the Merlins coordination effort and the answer was "we'll find out"?
#1I think that might depend on the directness of the link.. thinking of a woj(this one i'm rather certain exists) about if someone tried to control Mab with just that one name, Not having enough leverage. What Harry does to Toot is using his precise unbroken name, same as he latter does calling EK. EK had to know it was a trap, he just couldn't not show up to such a direct link to him. Notice most of the summonings that seem backed by 'choice of answering' are also using incomplete or common names for those things, or just plain asking as Harry seems to in DB of Lea.
What I wonder is if you call on something can more show up? Say, you call Odin, bad idea all around I know, can Kringle also make his power known? Technically different beings, summoning one doesn't necessarily summon the other does it? Now say instead you got ahold of Odin's original name, and summoned him by it, since he is both Odin and Kringle can he manifest power from either of them? Is it always a package deal that arrives? does using the source being differ from using a minor, but complete in and of itself, mask?