Use "find and replace". First of all replace all "fullstop+space" with
simply "fullstop". (That will reduce every gap of a single space between
sentences to only a fullstop between the sentences, but will also reduce
every gap of two spaces between sentences to simply a fullstop and a single
space.) Run the same replacement yet again. (This 2nd run wil result in
what were originally 2 spaces, but were reduced by the 1st application to
only one space, to no spaces and only the fullstop.) Run it again (and again)
until every sentence has only a fullstop between it and the next sentence,
with no space. Then, final step, run find & replace yet again, but this time
so that it finds every "fullstop" and replaces it with "fullstop+space+space".
If you know right from the start that EVERY sentence has only a
"fullstop+space" then all you need to do is run "find & replace" so that it
replaces every "fullstop+space" with "fullstp+space+space". The initial
description allows for all sorts of different spacings between sentences.
Of course, watch out for ellipsis. They can sometimes be a combination of
"fullstops" and you may not want to add an additional space after them..
