Subdivision Mirror Axis Conciseness

I started modeling a shark.
Now here is my problem which is also depicted on the picture.
The subdivsion mod can’t come to a concise union at the mirror axis.
I already added another edge loop along the mirror axis to give the modifier more flesh, but it didnt help.

Where is the problem?


It works when i push the extra edge loop right above the most inner mirror loop.
But then i would have doubles and that is not what i want.