To handle one pair of consecutive CRLF, you'd use one replace. For additional consecutive CRLF, you nest additional replace statements.
DECLARE @crlf char(2) = CHAR(13) + CHAR(10)
DECLARE @doubleCRLF char(4) = @crlf + @crlf ;
-- Two replaces handle 3 consecutive CRLF
UPDATE tablename SET field = REPLACE(REPLACE(field,@doubleCRLF,@crlf),@doubleCRLF,@crlf)