If I select all Notes in a bar, in which some overlap each other it seems to me impossible to get all (even all overlapping notes) selected:
The effect is, that when I paste them the ovelapping notes are missing which currently makes always some tedious corrections necessary which would not be necessary if all notes would be selected as I wanted it to do.
Is this Problem or any help to comme along with it already known?
The problem is caused by the fact that Dorico can’t select anything that you can’t see, and in this case where the eighth notehead is hidden by the half notehead, it’s not drawn. The workaround, which is a bit of a bore, is to click the beam to select all four notes under the beam, then click the stem of the down-stem half note to ensure its notehead is selected, then copy and paste or hit R or whatever.
Thank You for your clearing answer.
However the selection problem of hidden notes seem to me not that much consistent. It seem as if smaller notes do not benave in the same way when hiding others:
Perhaps It is perhaps possible to let all notes behave in the same manner (for instance like quater and eight notes. 
)
I would guess that when Daniel said “hidden” he didn’t mean what you think he meant.
Normally, computer graphics (including PDFs) are created in the same way as “painting”. You add “ink” which can cover up what is already on the “paper”, but you don’t erase anything.
So in the case of an “open” half note overlapping a “solid” 8th note, the 8th note head is not drawn at all, because the half note on top of it would still leave the black “ink” filling up the half note.
Also, in some music fonts, the “open” and “closed” note heads are not exactly the same shape either, so drawing both of them at “the same position” might create a mess.
On the other hand, two black notes can be drawn on top of each other without any problems, and a marquee selection will select both of them.
Trying to fix this by “painting” the center of the half note white (for example by creating a “multicolored” font using white “ink” as well as black), would be complicated, because staff lines, leger lines, etc., should still be visible in the “hole” in the note head - so you would have to draw the notes before drawing the staff lines, etc., and that might cause other problems.
Thank you Rob for these detailed explanations. I must admit I was like fahl5, wondering why there was a different behavior for the two examples he gave us.