To add to the description of the problem: I also have two clefs in the staff; it looks like it doubled ....

… I have six line in the lower staff for piano …

… I also have two clefs in the staff …

My first guess would be that the lower staff actually is two staves, with a vertical distance of 6pt.

Switch to the Staff Tool, and take a look at the lower staff in Scroll View, at a high zoom percentage.

If you can see two staff handles, then select one, and delete the staff.

thanks Peter, I am not an experienced user yet. I tried to follow your instruction and indeed there were two staves, but I did not succeed in deleting one of them: probably didn't do something right. Finally I copied the whole document to a new one and that also solved my problem.

Anyway thanks for your attention!

FYI: In recent versions of Finale, you delete staves in the Score Manager using the little "X" at the right of the staff info line.

… I tried to follow your instruction and indeed there were two staves, but I did not succeed in deleting one of them: probably didn't do something right …

When you are in the Staff Tool, you have access to the Staff menu.

When the staff handle is selected, go to

Staff menu > Delete Staves

or

Staff menu > Delete Staves and Reposition