Sorry for the late reply, but I cannot with good conscience leave
the issue as it was suggested using Krazy Glue to glue ivory keytops.
The method that I learned for replacing ivory keytops uses a special
clamp for the purpose.  I was able to obtain a set of them on eBay
but I also have seen them in the piano supply companies.

The clamp has two parts: a brass plate shaped like the fronts with a
lip for pressing it against the tail, and a steel clamp with a screw-on
top and a swivel foot that braces the clamp in place.  For the tails
it's the same clamp with a long brass bar.  To make sure that it
doesn't stick to the key, the brass should have a piece of masking
tape attached to it.

Carefully select the new ivory for the proper bevel, color and grain.
I went through thirty tops to select eight replacements.  The next step
is to clean the ivory; make sure that no splinters of wood remain on
the bottom and to make sure that no dirt is left where it will join
with the tail.  It's important to also make sure that both sides are
clean; assuming that the tail is intact, it should be clean from any
dirt at the seam as well.  Check the height of the new ivory with the
old tail; a very thin sheet of gauze may be needed to raise it to the
same level.

I have some antique glue that is ivory colored (a very hard block) that
I add hot water to dissolve, then place in a double boiler to keep it
hot, very much as one uses hot hide glue.  However, hot hide glue works
equally well: add some white powder (titanium oxide or zinc oxide) to
ensure the color will be white.

Working quickly as with any hot hide glue application, brush the glue
in place -- a very thin layer -- and place the replacement ivory in
place, then clamp it so that the special brace pushes it firmly against
the tail.  Make sure that it is straight and doesn't hang-out on the side.

Leave the clamp in place for at least one hour and do not disturb it
for a day.  When it's all finished, you may need to sand with finishing
paper to smooth the ivory so it's at the same level, and then the ivory
can be polished using a very gentle rouge.

I will also note that I managed to do this once without removing the
key from the keyboard -- just raising it a little.  It would be a real
delight to have to perform such a task on a Duo-Art removing first the
top action and the piano mechanism.

Best regards,
Eliyahu Shahar