It is my understanding that Dorico does not have a piano reduction feature as in Finale (not that it is perfect). The approved method is to Paste Special.>Reduced.
Is there any way to clean up such reduction after pasting? The source material I use tends to have many clunkers and I use a piano reduction to scan for those.
Unfortunately, I am getting unreadable stuff like this.:
is there a simple way to clean up the beaming and eliminate duplicate notes?
There isn’t a single tool that will simplify this task for you, no. There are several things that make reductions easier to work on in Dorico than other software:
As you’ve seen, it is especially difficult for software to choose how to place more than 2 voices on a staff without these collisions. Producing a good, playable keyboard reduction has always been a painstaking editorial task, and I believe it always will be, despite advances in AI.
On the other hand, if your reductions are mainly for proofing, you don’t have to edit anywhere near as carefully! Try reducing subsets of instruments onto more staves. If you’re just looking for clunkers, try this: Select everything, make them all eighth notes, and move them all to up-stem voice 1.
There is no simple route.
I would turn on View>Note and rest colours>Voice colours.
I would then filter a specific voice, then paste special> paste into voice… and choose a voice with the same rhythmic pattern. This progressively reduced the number of voices you are dealing with. (Both these operations are done using the right-click context menu)
I’ve had some success changing everything in one staff to one voice. Then I filter to see just the bottom or single note and change those I need to to a stems down voice.
I do have to say that Dorico desperately needs a way, similar to Sibelius change split-point feature, to take notes that shouldn’t really be in the LH and move to the RH (or vice versa).
Some people have suggested exporting to midi then importing back into Dorico to get the split point corrected.
I’ve never figured out a way to remove unison notes if there are other notes in the chord. (Otherwise, filtering as mentioned above works).
The beaming should be fine and match your layout, engraving and notation options.
Sometimes running the re-quantizing function can help get you on the way.
Another thing to consider is only using paste->reduce on less than the entire score then see if you really need to reduce the previously not reduced staffs. Or reduce certain families of instruments to a sketch staff, then do the final reducing. Even in Finale or Sibelius, the piano reduction options still required me a lot of editing to look right.
This is a bit of a radical suggestion, but may be worth trying: export it as a MIDI file and then re-import into the piano. The auto voice detection will try to find the best way of splitting splitting into voices. It might struggle with the way that it spans about 4 octaves, but I would be interested to see if it helps.
I didn’t realize until now that Dorico excelled at black MIDI. 
One stray “Wait for next break” will do it …
Where possible, try to reduce in chunks of music: if you can reduce sections in rhythmic unison separately from the sections that aren’t, you should get better results (ie Dorico can squish the music down into fewer voices).
Is there any what to:
When you’re reducing everybody at once, I don’t think so. But Lillie’s suggestion will help with both of those things.
I’ll politely reduce myself now 
and will phrase off 
What also works for now is to export to XML, import into Finale, and do piano reduction.
