Peter Tury said:
> However, the more serious problem is that it "forgets" such
> eliminations when it searches for `old' expansion in case of repeated
> calls of  `dabbrev-expand' (i.e. when the check "(eq last-command
> this-command)" in this function results t) what results in weird text
> in some cases.

I think I've got a fix for this.

The problem is that dabbrev--substitute-expansion alters EXPANSION
before using it in the substitution, but dabbrev-expand has no idea that
it's done this, so it assumes the unaltered EXPANSION is still valid and
saves it.

The patch makes dabbrev--substitute-expansion return EXPANSION, and
dabbrev-expand saves that version instead.

I considered making a new function to remove the whitespace and using it
in both dabbrev-expand and dabbrev--substitute-expansion, but returning
the value actually used strikes me as safer, in case anything else is
done to it.