Were your shortened page titles an exact match for the search term
Not usually. They used a more general term that covered all the words I took out when I shortened the titles.
and were there any other pages in the SERPs with the same title ?
Not exactly the same, but two or three other sites use titles that vary only by a word or two. Mostly we are talking synonyms there.
The shortening was not productive for me, rankings actually got worse (not sure if the continued fall was related to the title changes or Panda). I was hoping to offset the Panda effect, didn't work, so I changed the titles back after a month or two (I forget the exact timing).
After returning to the old style titles Google stopped modifying them.
Gradually there has been some recovery in those page rankings, can't say why with enough confidence to matter.